Summary Many autoantibodies involved in the pathogenesis of autoimmune thrombocytopenic purpura (AITP) are directed against epitopes on platelet glycoproteins (GP). These autoantibodies are a specific diagnostic characteristic of patients with AITP. In this study, the relative frequency of antibodies against GPs IIb/IIIa and Ib/IX was assessed in sera from 81 AITP patients with a glycoprotein-specific enzyme immunoassay (MAIPA assay) using monoclonal antibodies against these platelet GPs. All sera contained platelet-specific antibodies which had been detected by platelet immunofluorescence. Of the 81 antibodies tested, 58 (72%) reacted with at least one of the platelet GPs studied. Autoantibodies against GPIb/IX were as common as antibodies against the GPIIb/IIIa complex. The same ratio of specificities was observed on autologous platelets of an independent cohort of 29 patients. The epitope of three autoantibodies against GPIb/IX and of mab Gi10, a monoclonal antibody, which inhibits binding of these autoantibodies, was further characterized. Severity of thrombocytopenia was not related to the GP specificity of the autoantibody. The observation that in 23 (28%) of these sera the antigenic determinants could not be assigned to the glycoproteins under investigation suggests that platelet autoantibodies may react with other GPs or other membrane constituents, e.g. glycolipids.