The assessment of the extent of platelet activation in various thrombotic disorders is important to detect thrombotic tendencies and monitor antiplatelet therapy. Various methods (platelet factor 4: PF4, beta-thromboglobulin: beta-TG, CD62P, PAC-1, etc.) are used as markers of platelet activation. Platelet-derived microparticles (PDMP) are submicroscopic vesicles released by platelets during activation, and are used as a marker of platelet activation. PDMP are usually determined by flow cytometry. An enzyme-linked immunosorbent assay (ELISA) method in which PDMP are detected with the antibody of CD42a and CD42b, was recently reported. The PDMP sample for ELISA can be preserved, and standardization is also easy. It is expected to become an evaluation method of platelet function in vivo.