Platelets from 10 patients with Glanzmann's thrombasthenia (7 patients with type I and 3 with type II) and their 18 family members (11 parents, 6 siblings and one daughter) were analyzed by flow cytometry using 3 different commercially available FITC-labeled monoclonal antibodies. The amount of platelet GPIIbIIIa was calculated by using the ratio of the fluorescence intensity of the mean channel in comparison to normal platelets. The amount of platelet GPIIbIIIa was lower than 19% in 6 patients with type I and one patient with type II thrombasthenia. One type I patient had a 46.5% GPIIbIIIa amount as assessed using the monoclonal antibody TP80 (Nichirei Corp. Japan) which recognized GPIIb, although the other 2 antibodies showed an amount of less than 5%. One type II patient showed the following results: 30.9% (TP80), 28.2% (P2 antibody, Immunotech, France), and 3.9% (PLT1, Coulter Immunology, USA). The remaining type II patient consistently showed a normal amount of platelet GPIIbIIIa using all antibodies, appeared to be a variant form of thrombasthenia. The parents of type I patients had a significantly lower amount of platelet GPIIbIIIa compared to normal and 2 siblings of type I patients were diagnosed as heterozygotes. These findings suggest that Glanzmann's thrombasthenia is more heterogeneous than we have previously suspected, and that flow cytometric analysis using different monoclonal antibodies is a useful tool for identifying those heterogeneities and for the detection of type I carriers.