AbstractWe investigated the association of 2-glycoprotein I and P-selectin with platelet-derived microparticles in 48 patients with immune thrombocytopenic purpura and 20 normal controls using two-color flow cytometric analysis. In addition, anticardiolipin antibodies were detected by an enzyme-linked immunosorbent assay. Platelet microparticles from the patients showed a higher positivity for 2-glycoprotein I than those from the normal controls (23.115.4% vs. 5.33.1%, p0.01), but this positivity was not related to the presence of platelet-associated IgG or to the severity of thrombocytopenia. In the 18 patients with more than 20% P-selectin-positive microparticles, 2-glycoprotein I positivity was significantly higher than in the 30 patients with less than 20% P-selectin-positive microparticles (37.120.5% vs. 21.517.3%, p0.01). In addition, anticardiolipin antibodies were detected in eight patients, and they had a significantly higher level of 2-glycoprotein I-positive microparticles than the patients without such antibodies (42.022.9% vs. 22.618.9%, p0.05). Our results suggest that anticardiolipin antibodies activate platelets in immune thrombocytopenic purpura and cause the generation of microparticles rich in 2-glycoprotein I and P-selectin. These microparticles may then act to regulate coagulation abnormalities in patients with anticardiolipin antibodies.