Abnormal amounts of platelet-associated immunoglobulins (PAIg) and seropositivity for HIV were demonstrated in 18 haemophiliacs suffering from thrombocytopenia of varying severity. Among 34 non-thrombocytopenic haemophiliacs seropositivity for HIV was found in association with increased levels of PAIg in 18 patients,whereas increased amounts of PAIg were seen only in 4 of the 16 patients seronegative for HIV. In most of the haemophiliacs with thrombocytopenia, the PAIg represented all classes of immunoglobulins and in half of the cases all subclasses of IgG, while in 33 patients with autoimmune thrombocytopenic purpura (AITP), PAIg was IgGl and IgM in most cases. Our data show a difference in the pattern of PAIg in AITP patients and in thrombocytopenic haemophiliacs seropositive for HIV. We suggest that the thrombocytopenia in haemophiliacs is related to the polyclonal B cell activation also found in other patients seropositive for HIV and perhaps indicative of AIDS-related complex.