Fibrinolytic agents with higher specificity for fibrin in the thrombi and little systemic activation of the fibrinolytic system have been developed and tested in preliminary clinical trials of patients with acute myocardial infarction. The largest published experience available has been with recombinant tissue plasminogen activator, which seems to be more effective than streptokinase in lysing coronary thrombi. The acylated streptokinase-plasminogen complex BRL 26921 and pro-urokinase also gave promising preliminary results. All these agents, however, were accompanied by unexpectedly high incidence of systemic activation of the fibrinolytic system and by hemorrhagic complications with frequencies similar to those accompanying streptokinase. Hence, their superior clinical efficacy must be clearly proven before they are substituted for a more widely available and less expensive drug, such as streptokinase.