Lymphocytotoxic antibodies (LCT) directed against B and T cells were detected in 3 patients with AIDS, 7 patients with chronic lymphadenopathy syndrome and 13 of 17 symptom-free homosexuals. Five of these patients also had circulating cold agglutinins. Neutralization experiments have shown that LCT belonged to the IgM class. Elevated serum IgM was found in 15 of 23 (65%) patients with LCT, but in only one out of 4 patients (25%) who did not have LCT. No correlation of LCT to the risk factors or lymphopenia was observed. It may be concluded that many patients with AIDS and ARC synthesize complement dependent antilymphocyte antibodies which are cytotoxic to B and T cells.