The effect of strain-specific rabbit anti-serum on the ability of cultures of E. histolytica to ingest rabbit RBC has been studied. These sera have shown marked ability to inhibit the phagocytic activity of the homologous strain of E. histolytica and varying ability to inhibit the phagocytic activity of heterologous strains. One strain, K-9, appears to be quite different from six other strains when tested in this way. The other six strains have exhibited varying ranges of heterogeneity. It is suggested that these results indicate antigenic variation among strains of E. histolytica. Other experiments are necessary to provide final proof.