Fibrinolysis is a physiological process which aims at dissolving intravascular thrombi and is mediated by activation of plasminogen to plasmin. Streptokinase (SK) and urokinase (UK) are non-specific plasminogen activators. They have proved effective as thrombolytic agents, but their use is limited by the risk of haemorrhages due to systemic fibrinogenolysis. More fibrin-specific drugs have recently been developed. One is a tissue plasminogen activator (t-PA), the other is a urokinase precursor (pro-UK), also called single chain urokinase plasminogen activator (scu-PA). Genetic engineering techniques have resulted in the large-scale production of a "recombinant t-PA" (rt-PA) and a "recombinant scu-PA" (r scu-PA) for therapeutic use, notably in acute myocardial infarction. In vitro, these two drugs exhibit a thrombolytic activity that is equal to, or greater than that of SK or UK. In vivo, their fibrinogenolytic effect is less pronounced, and their thrombolytic effect greater than those of SK or UK. "Acyl-enzymes" have more recently emerged. These are inactive acylated SK-plasminogen complexes which progressively become effective in plasma after deacylation. So far, the most extensively studied of these complexes is BRL 26921 (anisoylated plasminogen streptokinase activator complex, or APSAC) which is administered by bolus intravenous injection. It is more thrombolytic than SK but produces systemic fibrinogenolysis to an equivalent degree. Injected intravenously (by infusion or bolus) during the first hours of a coronary infarction these three new thrombolytic agents have proved effective in promoting coronary reperfusion, with an early coronary patency rate of 70-75%.(ABSTRACT TRUNCATED AT 250 WORDS)