The authors characterized antiplatelet membrane antibodies in the sera of patients with idiopathic thrombocytopenic purpura (ITP) and cirrhosis of the liver. Antibodies were detected in five of the 22 patients with chronic ITP and in none of the eight patients with cirrhosis of the liver. The authors report a patient with chronic ITP in complete remission. Antibody to platelet glycoproteins (GP), with molecular weights of 55 and 49 kDa, was detected in the serum. The patient's immunoglobulin G (IgG) alone could cause the aggregation of platelet-rich plasma. Anti-GPIIb/IIIa antibody (LJ-CP8) inhibited the aggregation of platelet-rich plasma induced by the patient's IgG in a dose-dependent manner. The F(ab')2 of the patient's IgG had a synergetic effect on the aggregation of PRP induced by adenosine 5-diphosphate. This demonstrates that in ITP, the binding of IgG via its fragment of antigen binding site portion may activate platelets.