On the basis of electron microscopic observations, four types of cells apparently cytotoxic for antibody-coated chicken erythrocytes were identified in non-immune mouse spleen; monocytes, polymorphonuclear leucocytes, immature granulocytes and a type of lymphoid cell. Both phagocytosis and extracellular lysis of target cells were observed. Monocytes and polymorphonuclears were able to interact with the target cell by both mechanisms while the intermediary granulocytes and lymphoid cells were only capable of extracellular lysis. It is argued that these observations provide a morphological basis for the previous classification of antibody-dependent cytotoxic cells into myeloid and lymphoid cells (Greenberg et al., 1973b, 1975).