The gamma heavy-chain subclass of bound antiplatelet antibody was examined in six patients with autoimmune thrombocytopenic purpura (ATP) by a solid-phase radioimmunoassay. Monospecific antisera for gamma G1, gamma G2, gamma G3, and gamma G4 subclasses were employed in a “sandwich” technique, utilizing the binding of 126I-staphylococcal protein A. We have previously reported that serum antiplatelet antibody was restricted to be gamma G3 subclass in ATP. In contrast, all 4 IgG subclasses were found bound to platelets of ATP patients in the same distribution as that present in normal serum. It is suggested that the differences noted between serum antiplatelet IgG and platelet-bound IgG may represent different mechanisms of platelet injury.