A study was carried out on the presence of platelet-bound immunoglobulins, platelet-bound complement and serum immunoglobulin reactive with platelets in the blood of persons infected with HIV and those at risk of HIV infection. Platelet-bound immunoglobulins, predominantly IgG and IgM, but not complement, were demonstrated by immunofluorescence in 16 out of 16 patients with AIDS, in 5 out of 7 with AIDS-related complex/persistent generalized lymphadenopathy and in 7 out of 10 apparently healthy sexually active homosexual men, of whom 2 were anti-HIV1 seropositive. There was no correlation between the presence of platelet-bound immunoglobulins and either the platelet count or the level of circulating immune complexes. The specificity of the platelet-bound immunoglobulins and platelet-reactive immunoglobulins in the corresponding sera was studied. Platelet-bound immunoglobulins were eluted and then investigated for cross-reactivity with HIV. Both sera and eluates were tested for reactivity with cardiolipin and reactivity with the major target antigen in classical autoimmune thrombocytopenia, the GP IIb/IIIa complex. Of 17 eluates containing platelet-reactive immunoglobulins, 5 reacted with HIV-determinants but 2 out of 5 eluates that did not contain platelet-reactive immunoglobulins also reacted. Although anti-cardiolipin antibodies were detected in all sera, none of the 17 eluates reacted with cardiolipin. Moreover, sera and eluates, reactive with normal platelets, did not react with type-1-Glanzmann disease platelets. This indicates that the antibodies are directed against the glycoprotein IIb/IIIa complex of platelets. This could not be confirmed by immunoprecipitation or by immunoblotting, however. We conclude that the presence of platelet-bound immunoglobulins is common in HIV-infection but may also occur in persons at risk and that the nature of the auto-antibodies is not different from that of the auto-antibodies observed in classical ITP.