Three cases of suspected variant thrombasthenia patients (out of 10 cases of Glanzmann's thrombasthenia), who had significant amounts of platelet GPIIbIIIa, underwent flow cytometry to analyse the binding capacity of monoclonal antibodies against GPIIbIIIa to platelets. The monoclonal antibodies used in this study were as follows: PLT-1 and AP-2 recognizing the IIbIIIa complex; TP 80, P2 and AP-4 recognizing IIb:;AP-5 recognizing IIIa;OP-G2, which binds an epitope near the RGD binding site and 3F11. OP-G2 also recognizes conformational changes of activated platelets by increased binding. Case 1 platelets showed a binding capacity of 28-63% of that of normal platelets for TP80, AP-2, AP-4, and 3F11, but no binding to OP-G2. Case 2 platelets also showed 16-44% binding with TP80, AP-2, AP-4, AP-5, and 3F11, but no binding to OP-G2. These findings indicated the presence of structural abnormalities of the functional site of platelet GPIIbIIIa in cases 1 and 2. Case 3 platelets bound with all monoclonal antibodies normally, but normal increase in the binding of OP-G2 to platelets activated by thrombin or ADP was not seen, indicating a lack of activation of the fibrinogen binding site of platelet GPIIbIIIa.