Macrophages obtained from peritoneal exudate are effector cells for antibody-dependent cytotoxicity. This effector function is independent of phagocytosis, and it is not inhibited by EDTA or aminophylline. Cells functionally similar to those in peritoneal exudate can be obtained from the spleen, but a functionally different population is primarily responsible for cytotoxicity mediated by spleen cells. Effector function of these splenic cells is inhibited by EDTA and aminophylline. These splenic cells tend to adhere to plastic and their function is resistant to a high dose of x-irradiation.