1.Trophozoites from various cultures of strain 200 Endamoeba histolytica were evaluated as to their infectivity for rabbits.
2.When cultivated with a mixed bacterial flora, strain 200 produced a fatal ulcerative amebiasis in about 85 per cent of the inoculated animals as compared to 35 per cent when cultivated with a single bacterial associate, organism t.
3.Trophozoites from bacteria-free cultures, with T. cruzi as an associate, produced but one infection when inoculated into 24 animals.
4.The infectivity of trophozoites of E. histolytica grown with T. cruzi was restored after the amebae were re-established in LER culture with bacteria.
5.In vitro observations demonstrated that amebae grown in the absence of bacteria (amebae-T. cruzi cultures) became readapted to an environment with bacteria only with difficulty. Hence, this is offered as a possible explanation for the low infectivity of these amebae when inoculated intracecally into rabbits, although the possible influence of encystation on infectivity must be considered.