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The virulence of 70 isolates of Entamoeba histolytica was studied in the Wistar strain of albino rat by intracaecal inoculation of amoebae. The results were evaluated by the Neal scoring system, histopathological grading of ulcers and virulence indices. It was found that isolates from acute amoebic cases could infect and produce ulcers in significantly more animals than could the isolates from asymptomatic cyst passers. The virulence indices of isolates from acute cases were also significantly higher as compared to those from asymptomatic cyst passers. However, there was no significant difference between virulence indices of isolates from acute cases and non-dysenteric amoebic colitis. This study also showed that Neal scores failed to correlate with the type of pathology produced by the amoebae in the caecum. It is felt that virulence indices which depend upon the histopathological lesions should also be taken into consideration. The serological response of the patients tended to correlate with the virulence indices of isolates but only minimally. The results thus contradict a common belief amongst clinicians that higher levels of anti-amoebic antibodies reflect severe disease, as compared to low levels of antiamoebic antibodies.

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Nummary.The inreraction between Enlamoeba hislolyiica iropho-^oiies and colK o\' the immune system was studied in vitro. Both immune as well as non-iminuni; mononuctcar LCIIS (MNC) to some exienl were capable of exeriing a cylotoxic effcci on £. /imo/r/itw tropho/oitcs. Incorporation of immune serum directed against ihe tropho/oites in ihe reaction mixture resulted in a marked increase in the killing capacity of MNC. Highest killing was observed (96-7 t 1-04%) with MNC oblainect from animals immunii-ed with fraciion-!(l'-l) ol' the crude amoebic extract in association with atiti-K-l serutn. This cytotoxit event was found to be independent of complement involvement and required very low antibody coneentrations. This antibody-dependent cellular cytotoxicity against E. htstolyiicu in vitro was found to correlate well with ihe results of experiments reported earlier by us and others where a high degree ol' protection has been shown after vaccination of animals with F-1 followed by challenge with highly virulent strains of E. hislolyiica.