The effect of human leukocyte interferon (Hu INF alpha) on antibody-dependent cell-mediated cytotoxicity against antibody-treated chicken red blood cells was studied. Peripheral blood mononuclear cells were not significantly stimulated by IFN in ADCC. The lymphocytes freed from adherent cells, however, were more cytotoxic when IFN-treated, than without this treatment. The results might indicate that several potential effector cell types in the original mononuclear cell suspension competed for the targets and that only some of these effector cells became IFN-stimulated.