Thrombin inhibitors have recently advanced to the stage of preclinical testing as anticoagulants. However, little is known about the effects of these inhibitors on the enzymes of the fibrinolytic system. In the present study we evaluated the effect of two protein and two synthetic inhibitors of thrombin on tissue plasminogen activator (tPA), urokinase, and plasmin. We found that hirudin inhibited the amidolytic activity of plasmin but had no effect on tPA or urokinase. Antithrombin III inhibited plasmin and urokinase but had no effect on tPA. D-Phe-Pro-Arg-CH2Cl inhibited plasmin and tPA but had no effect on urokinase. Thromstop inhibited all three fibrinolytic enzymes: plasmin, urokinase, and tPA. Thus each thrombin inhibitor tested had different inhibitory effects on the fibrinolytic enzymes. These effects should be carefully considered when thrombin inhibitors are used as antithrombotic drugs.