The numbers of antibody-binding sites of platelet glycoprotein (GP) IIb/IIIa on circulating platelets were analyzed using 4 kinds of antibodies in 34 aplastic anemia (AA) patients, 20 idiopathic thrombocytopenic purpura (ITP) patients, and 14 normal controls. The numbers of antibody-binding sites of CD41, CD41a, CD41b, and CD61 on platelets of the AA patients were less than in the normal controls (p <0.001). In the ITP patients, the numbers of sites for CD41 and CD41a were less than in normal controls (p <0.05). There were significant positive correlations between CD41 and CD41a, CD41b, and CD61 in the 3 groups. There were significant negative correlations between CD41 and CD41b and between CD41a and CD41b in the normal controls, but not in the AA or ITP patients. In summary, the numbers of the 4 antibody-binding sites of GPIIb/IIIa on platelets of AA and ITP patients are different from those in normal controls. Measurements of the antibody-binding sites of GPIIb/IIIa are not necessary for the differential diagnosis of AA and ITP. However, the differences in correlations between the numbers of epitopes in AA and ITP patients suggest that the epitopes of GPIIb/IIIa are altered in these diseases.