Monospecific antiserum to an unusually stable Mr 50,000 plasmmiogen-activator inhibitor (PAI) purified from cultured bovine aortic endothelial cells was employed in conjunction with reverse fibrin autography to determine whether human platelets, serum, and plasma contain immunologically related inhibitors. Reverse fibrin autography revealed the presence of a Mr 50,000 inhibitor in the platelet and serum samples but not in normal plasma. However, a Mr 50,000 inhibitor was detected in plasma obtained from individuals with increased PAI activity. In each case, treatment of

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The maintenance of an intact vasculature is achieved through the complex interactions of the en dothelium, the cells present in the blood, and the en zymes, cofactors, zymogens, and inhibitors present in the blood plasma or stored within the various cells. Injury to the vascular endothelium results both in the rapid adhesion and aggregation of platelets at the site of the injury, and in the activation of the site-specific and highly regulated coagulation cascade. The conse quence of these multiple interactions is the formation of a platelet-fibrin plug and the subsequent cessation of blood loss. During the healing process, the clot must be resolved. This dissolution is accomplished by the fibrinolytic system which, like the coagulation system, is well-regulated, site-specific, and involves the interaction of many fluid and solid-phase compo nents.