Dissolution of blood clots (fibrinolysis) requires plasmin, a protease derived from the activation of plasminogen by tissue plasminogen activator (tPA). Both plasminogen and tPA are known to bind to the surface of platelets, where their interaction becomes greatly accelerated. Therefore, platelets are important promoters of fibrinolysis. Our laboratory has been examining the interaction of plasminogen and platelets with two types of assays: equilibrium binding experiments and rate measurements of platelet-dependent plasminogen activation. We have found that stimulation of platelets with thrombin increases high-affinity binding of plasminogen to the platelet surface and greatly accelerates the rate of plasminogen conversion to plasmin in the presence of platelets. Platelets that have been treated with carboxypeptidase B, which removes C-terminal lysines, lack high-affinity plasminogen binding and demonstrate much less ability to support plasminogen conversion.