A rapid and simple assay for cytopathogenicity of axenically cultivated Entamoeba histolytica has been developed employing baby hamster kidney (BHK) or mouse 3T3 cells conventionally tissue cultured. Three of the twelve amebal strains tested produced total destruction of the BHK cell monolayer in 2–3 hours, and these three strains are the three most virulent strains for the newborn hamster liver. Two additional strains were of moderate cytopathogenicity in vitro and of moderate virulence in vivo. Seven strains were of low cytopathogenicity and virulence. Within these three major groupings, however, the cytopathogenicity ranking was not entirely reproducible. The general correspondence of cytopathogenicity in vitro and virulence in vivo and the possibility of obtaining data within a few hours suggest such an assay may prove a useful tool in amebiasis research.