The clinical significance and detection methods for circulating activated platelets are reviewed. It has been recently demonstrated that three kinds of platelet granules, dense granule, alpha-granule and lysosome, have specific membrane proteins; granulophysin, GMP-140 (PADGEM) and CD63 antigen, respectively, and that these specific granule-membrane proteins become exposed on the surface of the activated platelets. It is believed that detection of circulating activated platelets, using monoclonal antibodies specific to these granule-membrane, may be reliable and diagnostic value in thrombotic and prethrombotic diseases. Preliminary clinical studies using 2T60, a newly developed anti-GMP-140 monoclonal antibody, was presented.