A comparison was made of susceptibility to lysis by human sera among five non-pathogenic and 11 pathogenic strains of Entamoeba histolytica already characterized into zymodemes. The nonpathogenic strains were found to be uniformly susceptible to lysis. Nine of 11 pathogenic strains, including five strains isolated from liver abscesses, were found to be resistant to lysis by serum under identical conditions. Resistance to complement-mediated lysis may be an inherent property of most pathogenic strains and may prove to be a necessary virulence factor for dissemination.