Two acyl-derivatives of urokinase, p-trimethylaminocinnamoyl-urokinase (TMAC-Uk) and p-guanidinobenzoyl-urokinase (GB-Uk),reactivating with different rates (k(reac) were 6 x 10(-4)/s and 6 x 10(-5)/s, respectively) were prepared. In comparison with free urokinase, acyl-activators were more stable in human plasma, and their stability increased with the decrease in the reactivation rate. Plasma clot lysis induced by all three agents was time- and dose-dependent, but acyl-activators caused a more prolonged fibrinolysis and lengthened lag-phase than free urokinase. Slowly reactivating GB-Uk induced the most long-lasting clot lysis, whereas free urokinase was more effective for the first 3 h. A combination of GB-Uk with low dose urokinase promoted the long-lasting clot lysis with the shortened lag-phase.