We have used chemical mutagenesis to isolate 10 independent phagocytosis-deficient mutants of Entamoeba histolytica from a highly virulent clone (strain HM1:IMSS). Phagocytosis-deficient mutants were characterized by several biologic properties related to amoebic pathogenic mechanisms: virulence, adhesion to red blood cells, cytopathic effect of live trophozoites on cell culture monolayers, ability of amoebic extracts to destroy cell culture monolayers, and growth in semisolid agar. All of these mutants were also deficient in in vivo virulence and in vitro cytopathic effect. Three were also deficient in adhesion, and the extracts of three others were unable to destroy cell culture monolayers. These findings suggest a strong correlation between phagocytosis and in vivo and in vitro cytopathogenicity. Both adherence of trophozoites to target cells and toxin activity of crude extracts of amoebae seem to be necessary factors for the expression of the virulence of this parasite. Traits responsible for growth in semisolid agar do not seem to be involved in virulence.