We compared the thrombolytic properties of recombinant staphylokinase (SAK) with those of streptokinase (SK), a tissue-type plasminogen activator (t-PA) and a urokinase-type plasminogen activator (u-PA) in the jugular vein thrombosis model in the rabbit in vivo and a circulating human plasma system in vitro. 50% thrombolysis was observed at 360 min after intravenous infusion into rabbits of 150 micrograms/kg of SAK or 500 micrograms/kg of t-PA, respectively. And the fibrinogen level in the blood was not affected by either agent. 50% clot lysis in vitro was observed at 120 min with 1.8 micrograms/ml of SAK, 22.1 micrograms/ml of SK, 2.1 micrograms/ml of t-PA, or 4.7 micrograms/ml of u-PA, respectively. All the plasminogen activators with the exception of SAK decreased the residual fibrinogen level in the circulating plasma at their moderate concentration for clot lysis. SAK had less influence on the plasminogen and alpha 2-plasmin inhibitor (alpha 2-antiplasmin) levels than the other plasminogen activators. These findings suggest that SAK is a potent fibrin-specific thrombolytic agent.