We have investigated the target antigens recognized by anti-platelet antibodies in patients with chronic idiopathic thrombocytopenic purpura (ITP) using an immunoblot procedure which could electrically separate the glycoprotein (GP) IIb/IIIa complex into GPIIb and GPIIIa. Various platelet proteins, having molecular weights of 167, 160, 145, 135, 124, 102, 92 and 80 kD, were recognized by circulating antibodies in 11 of 40 ITP patients. We identified the 145 kD antigen band, seen in two ITP patients, as GPIIb using thrombasthenic platelets as a source of target antigens. In one patient the anti-GPIIb antibody reacted with autologous GPIIb. These studies provide direct evidence for the presence of autoantibodies against GPIIb in some ITP patients.