We employed flow cytometry and monoclonal antibodies (MoAb) to study the surface membrane protein of shed particles (small vesicles, SV) that were released from vascular endothelial cells (EC) by agonists such as a Ca ionophore (A23187) and thrombin. After stimulation of EC by A23187, CD9 antigens disappeared entirely from the EC surface in a time- and concentration-dependent manner; they subsequently moved onto the SV surface. Von Willebrand factor (vWF) and P-selectin from Weibel-Palade (W-P) bodies were expressed rapidly on the EC surface after thrombin stimulation, but not on the SV surface. P-selectin may have some effect on maintenance of hemostasis on the EC surface. We demonstrated that the surfaces of SV and EC significantly supported prothrombinase activity and confirmed that A23187-induced SV from EC express binding sites for factors IXa and Xa. These results suggest that the SV are an important factor in a novel controlling mechanism of the coagulation system on the EC surface.