In a new 2-stage assay of platelet procoagulant activity (PCA), we first subjected gel-filtered platelets to adhesion on collagen (as a model of primary hemostasis) or plasma clots (as a model of preformed thrombus) for 30 minutes, and then the adherent platelets were supplemented with pooled, reptilase-treated, diluted plasma. Defibrinated plasma provided coagulation factors for assembly on platelet membranes without uncontrolled binding of thrombin to fibrin(ogen). Platelet adhesion to both surfaces showed modest individual variation, which increased at platelet densities that allowed aggregation. However, adhesion-induced PCA varied individually and surface-independently >3-fold, suggesting a uniform platelet procoagulant mechanism. Permanently adhered platelets showed markedly enhanced PCA when compared with the platelet pool in suspension, even after strong activation. The rate of thrombin generation induced by clot-adherent platelets was markedly faster than on collagen-adherent platelets during the initial phase of coagulation, whereas collagen-induced PCA proceeded slowly, strongly promoted by tissue thromboplastin. Therefore at 10 minutes, after adjustment for adhered platelets, collagen supported soluble thrombin formation as much as 5 times that of the thrombin-retaining clots. Activation of platelets by their firm adhesion was accompanied by formation of microparticles, representing about one third of the total soluble PCA. Collagen-adhered platelets provide soluble thrombin and microparticles, whereas the preformed clot serves to localize and accelerate hemostasis at the injury site, with the contribution of retained thrombin and microparticles.