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Karger Landes Systems, Basel, Switzerland, 1997, ISBN 3-8055-6497-X, soft cover. 195 figs., 15 tables, VIII+338 pp.

A study on the spatial distribution of two congeneric monogenean species Pseudodactylogyrus anguillae Yin et Sproston, 1948 and P. bini Kikuchi, 1929 on the gills of large size European eel Anguilla anguilla (L.) were conducted. Results were analysed with regard to: single-species infection, mixed infection and general occurrence of the parasites in the eel population. Statistical analysis revealed that the distribution of these species on the gill apparatus is fairly similar and the zones in which they occur more numerously coincide to a great extent. However, a number of differences were found. Both the possibility of simultaneous occurrence on the host and similar distribution of the above species on the eel's gills indicate the reciprocal tolerance of these parasites.

A new cucullanid nematode, Dichelyne mexicanus sp. n., is described from the intestine of three species of fishes, Agonostomus monticola (Bancroft) (Mugilidae, Perciformes) (type host), Ictalurus balsanus (Jordan et Snyder) (Ictaluridae, Siluriformes) and Cichlasoma beani (Jordan) (Cichlidae, Perciformes), from three rivers (La Maquina River, Veracruz; Chontalcoatlán River, Guerrero and Santiago River, Nayarit) in central Mexico. This species is characterised by the absence of a ventral sucker in the male (subgenus Dichelyne) and it differs from its congeners mainly in possessing very unequal and dissimilar spicules (left 0.465-0.768 mm and right 293-548 mm long), an asymmetrical gubernaculum, and two intestinal caeca. Another cucullanid nematode, Cucullanus caballeroi Petter, 1977, is reported from Dormitator maculatus (Bloch) (Eleotridae, Perciformes) from the La Palma and La Maquina Rivers and Balzapote stream, Veracruz, being briefly described and illustrated; this represents a new host record. Findings of D. mexicanus and C. caballeroi represent a new record of cucullanid nematodes from fishes in Mexican fresh waters.

Excretory/secretory products (ES), collected from in vitro cultures of muscle larvae (L1) of Trichinella spiralis (Owen, 1835) were examined for the presence of proteolytic enzymes. Several discrete proteinases in the size range of 25-55 kDa were identified by substrate gel electrophoresis and were characterised according to pH optima, substrate specificity and inhibitor sensitivity using azocasein assay. Serine, cysteine and metalloproteinases active at pH 5-7 were identified. The serine proteinases were found to predominate and some of them were found to be specific for the larval stage of the parasite. The results from the substrate analysis indicated the presence of collagenolytic and elastolytic activities. The proteinase activity was inhibited by IgG isolated from T. spiralis-infected mice, an observation of relevance to understanding host/parasite interactions and, ultimately, the development of anti-Trichinella vaccine.

The morphology of two Salvelinema Trofimenko, 1962 species, S. salmonicola (Ishii, 1916) and S. walkeri (Ekbaum, 1935), swimbladder nematodes of salmonids, was studied in detail, including scanning electron microscopy (SEM), on the basis of newly collected materials from Oncorhynchus masou, O. mykiss and Salvelinus malma from Japan and from Oncorhynchus clarki, O. kisutch and O. nerka from North America (Vancouver Island, B.C., Canada and Gulf of Alaska). Both nematode species proved to be morphologically very similar, differing substantially in the numbers and arrangements of egg filaments. Deirids were described for the first time for Salvelinema. Salvelinema iwana (Fujita, 1928), S. amemasu (Fujita, 1939), S. kosugii (Fujita, 1939), S. oncorhynchi (Fujita, 1939) and S. salvelini (Fujita, 1939) (= S. ishii (Fujita, 1941)) were synonymised with S. salmonicola. Comephoronema Layman, 1933 is re-erected as a valid genus related to Salvelinema.

Host responses against skin inhabiting monogeneans are commonly observed but the responsible immune mechanisms in the fish skin are insufficiently described. Based on recent knowledge of fish immunity and skin response mechanisms in mammals a model for the skin immunity in fish to monogenean infections is proposed. Important cellular components of the model are the epithelial cells, the mucous cells and leucocytes. The release of cytokines, e.g. IL-1, following mechanical or chemical injury of the epithelial cells, initiates a series of events leading to decrease of the ectoparasite population. Cytokines (e.g. IL-1, TNF, INF) are suggested to affect secretions from mucous cell and attract neutrophils and macrophages. Leukotrienes are probably involved in the inflammatory reactions. The subsequent production of humoral substances (among others complement factors and peptides) could be responsible for the antiparasitic response in the later stages of infection. Although non-specific factors dominate the response, the involvement of specific antibodies and lymphocytes cannot be excluded.

Eucoleus schvalovoj Kontrimavichus, 1963 (Nematoda: Capillariidae) is redescribed. The original description of this species was brief and inadequate in that it was based on just a few specimens removed from the Eurasian otter, Lutra lutra (Linnaeus, 1758) in the Khabarovsk region, USSR. Detailed morphological study of several specimens of E. schvalovoj from the oesophagus of L. lutra from Spain revealed new characters, above all in males, and allows for a better characterisation of this species. Since its original description E. schvalovoj has only been recorded in Spain.

The appearance of variations in the protein profile of Ostertagia ostertagi (Stiles, 1892) infective larvae (L₃), which were induced by hypobiosis triggering factors, was evaluated by means of SDS-PAGE and densitometric analysis. Area integration analyses of their protein profiles was carried out between 66 and 77 kDa. Important quantitative variations were identified in the protein levels of the induced larvae, where a 5.25 fold increase compared to the control was observed. Two 75.4 and 70 kDa protein bands were found which exceeded the control profile by 4.5 and 44 fold, respectively. This fact suggests that the changes brought about in the process of hypobiosis induction are restricted. This work demonstrates changes at a molecular level corresponding with biological changes induced by conditions causing O. ostertagi hypobiosis.

From November 1997 to June 1998, 3,118 specimens of Echinogammarus stammeri (Karaman, 1931) (Amphipoda) were collected from the River Brenta (Northern Italy) and examined for larval helminths. Larvae of Polymorphus minutus (Goeze, 1782) singly infected the hemocoel of 23 (0.74%) crustaceans; all these larvae were cystacanth stages. This is the first record of Polymorphus minutus in E. stammeri. Some cystacanths had their forebody and hindbody fully inverted. Parasites were bright orange in colour and each was surrounded by a thin acellular envelope. This envelope likely protects the developing parasite larva from cellular responses of the amphipod. Hemocytes were seen adherent to the outer surface of the envelope. The sex ratio among the parasitised E. stammeri was almost 1:1. All Polymorphus minutus larvae were central in the amphipod body, made intimate contact with host internal organs, and frequently induced a marked displacement of them. None of the infected females of E. stammeri carried eggs or juveniles in their brood pouch. In five hosts, Polymorphus minutus co-occurred with the cystacanth of another acanthocephalan, Pomphorhynchus laevis (Müller, 1776), a parasite of fish.

Isolation and characterisation of Plasmodium falciparum (Welch, 1897) soluble antigens from infected patient plasma, Western blotting, thermal stability and ELISA assays using hyperimmune IgG-antimalaria antibodies was the main objective of this work. A circulating antigen of approximately Mr 33-35 kDa with good specificity and antigenicity, in the plasma of malarial patients was shown. Heating at 100ºC did not destroy its antigenicity. When fractions highly enriched in the 33-35 kDa proteins were used in ELISAs, a seroreactivity in plasma obtained from primary-infected individuals was found. Controls from normal patients were always negative. The antigenic characteristics suggest that it may be included within the group of new described Plasmodium soluble antigens.

Nine Pteromylaeus bovinus (E. Geoffroy St. Hilaire) (Euselachii: Myliobatinae) from the Tunisian coast were examined for ectoparasites during 1995, 1996 and 1997. A monogenean found on the gills of seven host specimens is described as a new species and a new genus, Heliocotyle, is proposed to accomodate it. The presence of a single sclerotized haptoral structure on the dorsal surface of the haptor leads us to place this species in the Heterocotylinae Chisholm, Wheeler et Beverley-Burton, 1995. The genus Heliocotyle is characterized by a haptor with only one dorsal structure and seven loculi, six being subdivided by pseudosepta. Heliocotyle kartasi sp. n. is the type and the only species in this genus.

Spermatozoa of the monogenean Heterocotyle capricornensis Chisholm et Whittington, 1996 are long and filiform, comprising an elongate nucelus, probably a single elongate mitochondrion and two incorporated axonemes, one of which is shifted with respect to the other. The shift results in a region at each end of the sperm where only one axoneme is present, accompanied by the nucleus and mitochondrion at one end and the nucleus and/or mitochondrion at the other. By taking note of the direction of dynein arms on the axonemal doublet microtubules, each axoneme is identified and followed from beginning to end. No basal bodies remain in mature sperm but the main nuclear end is interpreted as proximal/anterior based on the final stages of spermiogenesis. A group of four or five cortical microtubules from the spermatid zone of differentiation persists in mature sperm, but is not closely associated with a region of extracellular matrix, as it is in other monocotylids. The sperm structure is compared with that of other monocotylids and the phylogenetic implications are discussed. Aberrant sperms in one individual were folded and fused along much of their length.

A new species Chonopeltis liversedgei sp. n. of the endemic African genus Chonopeltis Thiele, 1900 (Crustacea: Branchiura) is described. This ectoparasite inhabits the branchial chamber of its mormyrid host Mormyrus lacerda Castelnau, 1861 and was collected from three localities in the Okavango River and its inland Delta in Botswana, Southern Africa.

Many animals obtain reliable information about potential mates, including whether they are parasitised or not, mostly from olfactory cues in urine. Previous experiments with rodents have shown that females can detect parasites in males that are potentially transmissible during copulation, so that females can directly avoid infection by discriminating against parasitised males. Here, using choice tests, we examine whether female rats can distinguish males infected with the tapeworm Hymenolepis diminuta Rudolphi, 1819, a parasite with a complex life cycle and thus not directly transmissible among rats. Female rats tended to spend more time investigating the urine of non-parasitised males than that of parasitised males. The magnitude of the parasite burden in the infected males had no effect on the females' preference for the non-parasitised males. We also found that parasitised males had lower testosterone levels in their blood than non-parasitised males. These results suggest that females use cues in male urine reflecting either the presence of the parasite and/or lower testosterone levels to avoid parasitised males and possibly secure resistance genes for their offspring.

Plasmodia of a Henneguya species measuring 70-900 µm and exhibiting season-dependent stages of development were detected throughout a three-year study on gill myxosporosis of Lake Balaton pikeperch (Stizostedion lucioperca (L.)). Sixty-five out of 160 fish (41%) examined in the period of study were infected by the parasite. Infection was the most prevalent (48%) among pikeperch specimens exceeding 40 cm in length. The highest prevalence of infection (58%) was recorded in 1995-1996 while the lowest (30%) in 1996-1997. The youngest plasmodia appeared in April, and started to develop within the capillaries of the secondary lamellae of the gill filaments. The round or ellipsoidal plasmodia which continued their gradual growth in the subsequent months of the year achieved a size of 800-900 µm by the late autumn months, but remained in intralamellar location throughout the developmental cycle. Mature spores developed in the plasmodia by the end of winter. On the basis of their shape and size, the spores were identified as Henneguya creplini (Gurley, 1894). However, because of the uncertain taxonomy of species assigned to the genus Henneguya the taxonomic position of the parasite requires further study. The host reaction consisting of epithelial proliferation and granulation tissue formation starts around the infected secondary lamella only after the maturation of spores and the disruption of plasmodia.

The parasitic copepod fauna of 182 specimens of Mustelus schmitti Springer from the coast of Mar del Plata, Argentina was investigated. Three species of parasitic copepods were identified: Nessipus orientalis Heller, 1865 from the buccal cavity, Perissopus oblongus (Wilson, 1908) from the edge of pectoral, pelvic, dorsal, anal and caudal fins and in claspers, and Lernaeopoda galei Krøyer, 1837 from the base of the pectoral fins. N. orientalis was most common being present the entire year, while P. oblongus and L. galei occurred seasonally with low prevalence and mean intensity. There were differences in the site of infection by these copepods and variations in the relationship between prevalence and mean intensity and host size and seasonality. These parameters were unrelated to host sex. Our data suggest that the structure of this parasite community is a result of a complex of biotic and abiotic factors, such as temperature, spawning and breeding preferences of the host, and overlapping in the distribution of different shark species. This is the first report of N. orientalis in Argentinean waters.

Latent toxoplasmosis is the most widespread parasite infection in developed and developing countries. The prevalence of Toxoplasma gondii infection varies mostly between 20 to 80% in different territories. This form of toxoplasmosis is generally considered to be asymptomatic. Recently published results, however, suggest that the personality profiles of infected subjects differ from those of uninfected controls. These results, however, were obtained on non-standard populations (biologists or former acute toxoplasmosis patients). Here we studied the personality profiles of 191 young women tested for anti-Toxoplasma immunity during gravidity. The results showed that the differences between Toxoplasma-negative and Toxoplasma-positive subjects exits also in this sample of healthy women. The subjects with latent toxoplasmosis had higher intelligence, lower guilt proneness, and possibly also higher ergic tension. The difference in several other factors (desurgency/surgency, alaxia/protension, naiveté/shrewdness, and self-sentiment integration) concerned changes in the variances, rather than the mean values of the factors.

From January 1992 to December 1993, a total of 2158 fish, namely Oreochromis leucostictus (Trewavas, 1983), Micropterus salmoides (Lacepede, 1802), Tilapia zillii (Gervais, 1848) and Barbus amphigrama (Boulenger, 1902) were sampled from thirteen stations on Lake Naivasha, Kenya, using a fleet of gill nets and examined for helminth parasites. The prevalence of infection due to cystacanths of an acanthocephalan, Polyacanthorhynchus kenyensis Schmidt et Canaris, 1967 among parasitized O. leucostictus ranged from 30.4 to 86.9%; among T. zillii from 4.1 to 77.7%; in M, salmoides from 20 to 50%; and in B, amphigrama from 5.8 to 100%. In 735 hosts belonging to the above four species, a total of 4198 immature specimens of P. kenyensis were recovered. All cystacanths were found in extraintestinal sites, either free within the fish body cavity or encysted within the host visceral organs. There was no significant variation in the prevalence of the parasite within months (P > 0.001). Host sex ratio was significant (P < 0.001) in favour of male T. zillii, and also highly significant (P < 0.001) in favour of male O. leucostictus. Moreover, in this fish, prevalence of infection was observed to increase with the increase in the size of the fish. Among infected M. salmoides, there was no significant departure from a 1 : 1 sex ratio.

Three amoeba species were isolated from 3 out of 193 farmed tilapias, Oreuchromis niloticus (L.), screened for the presence of free-living amoebae in parenchymatous organs. Hurtmannella vermiformis Page, 1967 and Rosculus ithacus Hawes, 1963 were isolated from the kidney tissue. The third strain isolated from the liver shared morphological features of Mayorella and Platyamoeha spp. and therefore its taxonomic position has not been determined as yet. Pathogenicity of cloned strain of H. vermiformis was proved in two fish hosts.

Fifty-day-old fry of tilapia hybrids (Oreochromis aureus x niloticus) were placed in aquaria containing sediment with oocysts of Eimeria (sensu lato) vanasi Landsberg et Paperna. In the first 29 h after exposure sporulated oocysts in the stomach and free sporozoites in the gut could be found in examined fish. By 7 to 56 h after exposure, sporozoites, with their characteristic crystalloid body, were detected in intraepithelial lymphocyte-like and other leucocyte-like cells, but never in the epithelial cells. Infected cells were confined to the epithelial layer and did not enter the lamina propria. Within this time, some of the sporozoites divided by endodyogeny, once or twice in succession, to form daughter sporozoites. The parent's sporozoite crystalline body was divided between the offspring of the primary and secondary divisions.

Ectoparasitic Prototransversotrema steeri Angel infected Acanthopagrus australis (Günther) in western (W. Bay) and southern (S. Bay) Moreton Bay with greatest abundance in winter (June - August), and none in summer (December - February). This confirms previous observations on seasonality of P. steeri from a small estuary in New South Wales. Transversotrema licinum Manter, which is a new host and geographical record, infects A. australis in W. Bay but not S. Bay, with large abundances in summer and winter, lowest abundance in spring. Increased size of P. steeri in winter in S. Bay could be explained by increased size of fish sampled since fish length and size of each parasite species were correlated positively. Transversotrema licinum increased in size from summer to winter irrespective of fish size. Number of eggs in utero and parasite size were correlated for both species; worms in mid-size range had most eggs, and P. steeri had more eggs (average = 19.2, 0-80) than T. licinum (3.3, 0-21); eggs were similar in length. Number of eggs per parasite, adjusted for parasite size, was greatest in autumn (P. steeri) or summer (T. licinum), i.e. early in infection period. Spawning migration to eastern Moreton Bay did not influence number of eggs per parasite.

In most amoeboid cells, the main protein involved in motility is actin. Nematode sperm are an exception, and their amoeboid motility is based on major sperm protein (MSP). We have studied the localization of actin and MSP in spermatids and spermatozoa of Graphidium strigosum (Dujardin, 1845), a species which has elongate male germ cells in which organelles are easily identified. Electrophoreses of G. strigosum sperm proteins indicate that the main protein band, about 15 kDa in molecular weight, is specifically recognized by an anti-MSP polyclonal antibody developed against MSP of Caenorhabditis elegans (Burke and Ward 1983). Actin is present in small quantities. Immunocytochemical observations reveal that actin and MSP have an identical localization in precise areas of the male germ cells. Spermatids are labelled as dots around a central unlabelled zone, and spermatozoa are labelled only at the level of the anterior cap. Observations in G. strigosum are similar to that previously obtained in Heligmosomoides polygyrus (Mansir and Justine 1996). Co-localization of actin and MSP in the anterior cap of the spermatozoon, the region associated with pseudopod production, does not demonstrate directly that actin is involved in amoeboid movements, but shows that the role of actin in the cytoskeleton of nematode sperm should be re-investigated.

Three-spined stickleback (Gasterosteus aculeatus L.), nine-spined stickleback (Pungitius pungitius (L.)) and flounder (Platichthys flesus (L.)) are widespread teleosts, which all have behaviours involving migration between freshwater and brackish/sea water environments. Their importance in dispersal of the freshwater monogenean Gyrodactylus salaris Malmberg, 1957, which causes heavy losses of Atlantic salmon (Salmo salar L.) parr in infected Norwegian rivers, was tested indirectly by their susceptibility and resistance to the parasite in laboratory experiments. Gyrodactylus salaris attached to the three fish species, but no parasite reproduction was observed. The infections were eliminated after a maximum of 3 days on flounder, 6 days on nine-spined stickleback, and 8 days on three-spined stickleback. Thus these fishes are innately resistant to G. salaris, and are therefore of no importance concerning the population dynamics of G. salaris in freshwater systems. However, attachment of parasites indicates that these fish species may function as transport hosts, and theoretically play a part in the dispersal of G. salaris in nature.

This study was undertaken to attempt to identify correlations between microsporidial seroprevalence data in man, clinical diseases and groups of people at the risk of HIV/AIDS infection. Groups of patients were selected according to the predilection of members of the genus Encephalitozoon for nervous and kidney tissue. Female prostitutes and alcohol and intravenous drug abusers were selected as groups at risk of HIV/AIDS infections. A total of 401 samples of human sera were examined for the presence of antimicrosporidial IgG antibodies by ELISA test with a titre of 600 considered borderline positivity. The highest occurrence of antimicrosporidial antibodies was found in the groups of alcohol abusers (16 % from 43 patients), intravenous drug abusers (11 % from 9 patients) and prostitutes (10 % from 80 women) for E. cuniculi antigen and in the groups of psychiatric patients (14 % from 44 patients), malaria patients (11 % from 38 patients) and alcohol abusers (7 % from 43 patients) for E. hellem antigen. The occurrence of specific antibodies of the six examined diagnostic units (glomerulonephritis chronica, pyelonephritis chronica, schizophrenia, dementia, multiple sclerosis and cerebral stroke) was statistically significant only in patients with pyelonephritis chronica and dementia (p < 0.05). No cases of microsporidial infection were found among the female prostitutes by parasitological examination, although one case of giardiasis was identified. Sera of patients with high anti-E. cuniculi and anti-E. hellem antibodies (titres in ELISA of 600 and above) were confirmed by Western blot using E. cuniculi and E. hellem polypeptides, respectively. These results suggest that the examined patients could show residual antibodies from past or latent infections.

Polymerase chain reaction (PCR) techniques have been developed for the detection of microsporidian DNA in different biological samples. We used sequence data of the rRNA gene for the identification of Enterocytozoon bieneusi, Encephalitozoon intestinalis, E. cuniculi, and E. hellem in different biological samples of HIV-infected patients by PCR, Southern blot hybridization, restriction endonuclease digestion analysis, cloning, and comparative genetic sequencing. One primer pair was used for amplification of the entire small subunit (SSU)-rRNA gene of E. bieneusi, E. intestinalis, and E. hellem from samples with electron microscopy confirmed infection. The amplified 1.2 kb SSU-rRNA gene fragments were ligated into a pMOSBlue T-vector, transfected into pMOSBlue competent cells, and were used as positive controls. Several primer pairs and hybridization probes were used to amplify and identify microsporidian DNA from different samples. Light microscopical examination of samples was performed in all patients and transmission electron microscopy was done on a subset of patient samples. DNA products were obtained from all samples with confirmed microsporidial infections. The identity of the DNA fragments was determined by Southern blot hybridization or by restriction endonuclease digestion analysis or by DNA sequencing. The results show that PCR is a reliable and sensitive indicator for the presence of microsporidian DNA in different biological samples of HIV-infected patients. PCR can be used further for species differentiation of microsporidia, even between species which cannot be differentiated by light and/or electron microscopy.

A total of 2,086 specimens of Engraulis anchoita Hubbs et Marini (Pisces: Engraulidae), from waters off Argentina and Uruguay were examined for parasitic digeneans. Four species of digeneans were found, Parahemiurus merus (Linton, 1910) (adult) in the stomach; Lecithochirium microstomum Chandler, 1935 (non-ovigerous and ovigerous juveniles) in the swimbladder; Cardiocephaloides sp. (metacercaria) in the eyes and Rhipidocotyle sp. (metacercaria) in the musculature. These four species are recorded for the first time as parasites of E. anchoita.

The survival of Encephalitozoon cuniculi Levaditi, Nicolau et Schoen, 1923 spores suspended in distilled water and exposed at defined temperatures was investigated. Infectivity of E. cuniculi spores was tested by inoculation of SCID mice. There was no marked loss of infectivity of spores stored at 4°C for two years or frozen at -12°C and -24°C for 1, 8, and 24 h. Although there was a remarkable loss of infectivity, spores remained infective after freezing at -70°C for 1 and 8 h. Heating at 60°C and 70°C for 5 min and 1 min, respectively, rendered the microsporidia non-infective. These findings demonstrate that E. cuniculi spores suspended in water can survive freezing temperatures but lost infectivity in water that reached a temperature of 60°C at 5 min.

Microsporidia are eukaryotic, obligate intracellular organisms defined by their small spores containing a single polar tube that coils around the interior of the spore. After appropriate stimuli the germination of spores occurs. Conditions that promote germination vary widely among species, presumably reflecting the organism's adaptation to their host and external environment as well as preventing accidental discharge in the environment. It appears that calcium may be a key ion in this process. Regardless of the stimuli required for activation, all microsporidia exhibit the same response to the stimuli, that is, increasing the intrasporal osmotic pressure. This results in an influx of water into the spore accompanied by swelling of the polaroplasts and posterior vacuole. The polar tube then discharges from the anterior pole of the spore in an explosive reaction and is thought to form a hollow tube by a process of eversion. If the polar tube is discharged next to a cell, it can pierce the cell and transfer the sporoplasm into this cell. Polar tubes resist dissociation in detergents and acids but dissociate in dithiothreitol. We have developed a method for the purification of polar tube proteins (PTPs) using differential extraction followed by reverse phase high performance liquid chromatography (HPLC). This method was used to purify for subsequent characterization PTPs from Glugea americanus, Encephalitozoon cuniculi, E. hellem and E. intestinalis. These proteins appear to be members of a protein family that demonstrate conserved characteristics in solubility, hydrophobicity, mass, proline content and immunologic epitopes. These characteristics are probably important in the function of this protein in its self assembly during the eversion of the polar tube and in providing elasticity and resiliency for sporoplasm passage.

Maritrema orensensis sp. n. (Digenea: Microphallidae) from Balneario Orense, Buenos Aires province, Argentina, is described. Adults of M. orensensis were collected from the intestine of the kelp gull Larus dominicanus Lichtenstein, 1823 (Aves: Laridae) and differ from adults of other related species, principally, in the shorter intestinal caeca and the incomplete ring of vitelline follicles.

We determined the surface-associated proteolytic activity in three Entamoeba histolytica Schaudinn, 1903 strains (monoxenic HM1, axenic HM1, and HK9) of known virulence and its relationship with collagenase activity. Both activities were also determined in axenic HM1 amoebae trophozoites which were sensitive and resistant to complement-mediated lysis. Surface proteolytic activity was determined in glutaraldehyde-fixed E. histolytica trophozoites, which degraded the insoluble substrate, hide powder azure, and cleaved the human immunoglobulin G heavy chain in a time-dependent fashion, at neutral pH, in presence of 2-mercaptoethanol as cysteine protease activator. Surface proteolytic activity was strain dependent: monoxenic HM1 > axenic HM1 > axenic HK9. This activity correlated with collagenolytic activity (p < 0.05). Acquisition of resistance to complement-mediated lysis by axenic HM1 strain did not modify either surface proteases or collagenase expression. Our results suggest that this surface proteolytic activity could be used as an in vitro virulence marker for E. histolytica.