SUMMARYWe have looked at the effect of in vivo cortisone acetate treatment on effector cells for antibody-dependent cell-mediated cytotoxicity in mice and rats, using both chicken erythrocytes and the mouse lymphoma cell line AKR.A as target cells, since the AKR.A cell line is susceptible to antibody-dependent cell-mediated cytotoxicity killing only by the lymphoid effector cell, whereas a wide variety of effector cells will lyse chicken erythrocytes in the presence of antibody. The lymphoid K cell, detectable in rat spleen and blood, was unaffected by steroid treatment sufficient to cause lymphopenia, whereas splenic anti-chicken erythrocyte cytotoxicity of whole spleen and of phagocyte-free spleen was depressed in mice and rats. The greatest suppression was seen with non-phagocytic mouse spleen, and may have been in part attributable to steroid-induced redistribution of the effector cell(s), since the cytotoxic capacity of nonphagocytic bone marrow cells was increased by 70% at a time when the activity in spleen was 25% of normal.