SummaryWhen present during clot formation, tissue plasminogen activator (TA, porcine) is strongly bound to fibrin, in marked contrast to urokinase (UK, human). The amount of bound activator is approximately proportional to the total concentration of activator present, suggesting the formation of a dissociable complex between activator and binding sites on the fibrin. Frozen sections of plasminogen-free fibrin clots prepared in the presence of TA or UK, studied with the histochemical fibrin slide technique, showed sites of fibrinolytic activity related to fibrin strands in the clot. The activity caused by TA was more uniformly distributed along the fibrin strands than that caused by UK.

Human recombinant tissue-type plasminogen activator (rt-PA) has been shown to be an effective and safe agent for coronary thrombolysis in patients with acute myocardial infarction. However, thrombolysis is associated with a high rate of acute reocclusion after discontinuation of rt-PA. The goals of the present study were to assess whether reocclusion after thrombolysis is caused by intracoronary platelet aggregation and to determine the role of thromboxane A2 (TxA2) No. 3, 678-684, 1988. CORONARY THROMBOLYSIS has recently received great attention for its potential reduction of the1 extent of necrosis after myocardial ischemia. Thrombolysis can be accomplished by infusion of streptoki-

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