Washed human blood platelets contain a clottable protein that is similar to, if not identical with, plasma fibrinogen (1-4). After incubation with trypsin under appropriate conditions, platelets remain morphologically intact but no longer contain clottable protein (5). Trypsinized platelets, unlike normal platelets, are not aggregated by fresh serum or by a solution of thrombin and calcium chloride. When resuspended in plateletfree plasma or in a buffered solution of fibrinogen containing glucose, trypsinized platelets produce retraction of clots formed by thrombin; during the formation of the clot and its subsequent retraction, trypsinized platelets aggregate and undergo the usual changes of viscous metamorphosis (5). These observations suggest that fibrinogen on the surface of platelets is involved in the reaction of platelets to thrombin. This reaction does not consist simply of the coagulation of fibrinogen with entrapment of platelets in the fibrin mesh. Under usual conditions divalent cations are required for thrombin-induced platelet aggregation but not for clotting of fibrinogen (5, 6). Electron photomicrographs of platelet aggregates show no striated fibrin strands between adherent