This study demonstrates that a cytotoxic serum reactivity not requiring the presence of complement appears in the sera of patients with acute myelogenous leukemia. The reaction is detected upon short-term incubation of sera in vitro with autochthonous mononuclear white blood cells from peripheral venous blood of patients at the acute stage of the disease. This reactivity was demonstrated in 18/18 patients. Generally, the cytotoxicity was low in patients at the acute stage of the disease, but increased after chemotherapy and reached the highest level at the onset of clinical remission or just before. No cytotoxicity could be demonstrated against autochthonous remission white blood cells. The serum activity could be absorbed and eluted from protein A-sepharose CL-4B and was recovered in the 7S-fraction of the sera after gel filtration on Sephadex G-200 and ion exchange chromatography. This indicates that the demonstrated cytotoxicity is due to immunoglobulins of IgG-class. It is believed that Fc-receptor-bearing cells present in the target cell preparations function as effector cells. The reaction is designated antibody-associated cellular cytotoxicity (AACC).