The effect on amebal virulence of Escherichia coli, Clostridium symbiosum and a mixture of lactobacilli was investigated. Amebas from HM1 and HK axenic strains were incubated with a single bacterial strain or lactobacilli for short, intermediate, or long periods and analyzed for their erythrophagocytosis, hemolytic activity and cytotoxicity in vitro and hepatic abscesses formation in vivo. It was found that in vitro virulence was enhanced by all studied bacteria in HK9 but not in HM1 strains, which only showed an increase in cytotoxicity. The virulence in vivo was solely increased in trophozoites of HM1 strain by all tested bacteria.