Compositions comprising membrane-derived and synthetic microparticles that induce platelet aggregation and are useful for treating bleeding disorders, particularly those involving platelet dysfunction. Microparticles include endothelial derived microparticles (EMP), platelet derived microparticles (PMP), erythrocyte derived microparticles (RMP), and synthetic microparticles (SMP), and are used to treat disorders such as thrombocytopenia caused by chemotherapy.