We have found that Western blots (WBs) of whole platelets exposed to normal autologous or homologous sera commonly have bands at 90 to 95 (95) Kd, and less often at 100 to 110, 80 to 85, 60 to 75, and 50 to 60 Kd when developed with antiglobulins. The percentages of normal sera producing a 95-Kd band with anti-immunoglobulin G (IgG), -IgA, and -IgM are 85, 50, and 30, respectively. Antiglobulin reagents alone also produce background bands on WBs that we have shown correspond to levels of platelet-associated Igs (PAIgs) or their derivatives. Titers of 95 Kd-reactive IgG in normal sera range from 10 to 1,280 (85% less than or equal to 50), and the reaction appears to be partially F(ab')2- mediated. The 95-Kd protein is internal and differs in many respects from surface glycoproteins IIIa, IV, and V of similar apparent molecular weight. In thrombocytopenic patients there was no correlation between severity of thrombocytopenia or PAIgG of platelet eluates and corresponding serum titers of 95 Kd-reactive IgG. Some WB reactions previously reported as evidence of autoimmunity may represent normal variations in reactions of Igs with internal platelet proteins. These reactions may be immunospecific or analogous to nonspecific, partially F(ab')2-dependent binding of Igs by certain bacterial proteins.