In the present study we have investigated and compared in vitro the specific complement (C) activating activity of three metacestode preparations of Echinococcus granulosus. Extracts from hydatid cyst fluid (HCF-ext), protoscoleces (PSC-ext) and hydatid cyst membrane (HCM-ext) activated human C producing C3 conversion and generating the C5b6 complex and the terminal C complex (TCC). HCM-ext showed much lower C activating activity than PSC-ext and HCF-ext. Moreover, its ability to generate C5b6 and TCC was lower than its ability to convert C3. On the other hand, PSC-ext and HCF-ext proved to be good C activators when their specific C activating activities were compared with that of inulin. However, PSC-ext produced lower levels of TCC than those produced by HCF-ext, in spite of the fact that both produced practically the same levels of C3d and C5b6. These results may be consistent with the existence of several mechanisms of C modulation involved in the defence of the parasite against host C damage.