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We have attempted to determine whether amebal viruses are capable of viral conversion of the virulence of Entamoeba histolytical. Amebae of little or no virulence for the newborn hamster liver were infected with viruses obtained from four amebal strains of different virulence. Cultures of surviving amebae were increased in virulence by viruses from amebae of high, moderate, or low virulence, but decreased in virulence by the virus of the second most pathogenic amebal strain. Sequential infection with two viruses did not produce consistent cumulative effects on virulence. The changes in virulence noted are considered to be the unpredictable result of either selection pressures exerted by the lytic virus infection or possibly viral alterations of the amebae.