In persons with AIDS or at risk from AIDS, autoantibodies against platelets and granulocytes were frequently detected. Platelet-bound immunoglobulins were demonstrated by immunofluorescence in all 16 patients with AIDS, in five out of seven patients witn AIDS-related complex/persistent generalized lymphadenopathy (ARC/PGL) and even in seven of 10 healthy sexually active homosexual men. Granulocyte-bound immunoglobulins were found by immunofluorescence in 12 of the 16 AIDS patients, five of the seven patients with ARC/PGL and two of the 10 symptomless men. Red cell bound immunoglobulins were not detected. All patients with AIDS and ARC/PGL and three of the symptomless men were seropositive for human immunodeficiency virus (HIV). The platelet- and granulocyte-bound immunoglobulins could be eluted in 93% and 67% of the cases, respectively. This indicates that specific autoantibodies, rather than circulating immune complexes, which were frequently increased, accounted for the findings. There was no relation between the serological findings and the platelet and granulocyte counts. We conclude that autoantibodies against platelets and granulocytes are common in patients with AIDS and those at risk.