Abstract: A novel protease (designated as SW-1) purified from the fermentation broth of Streptomyces sp. strain Y405 showed fibrinolytic activity. In vitro, on plasminogenfree fibrin plate which had been heated at 85℃ for 30 minutes to eliminate plasminogen, SW-1 showed the same fibrinolytic activity as the mixture of SW-1 and plasminogen. Without adding plasminogen into the fibrin clot tube SW-1 showed the same effect as with plasminogen(P>0.05). The results might indicate that SW-1 is a fibrinolytic enzyme which degrades fibrin directly, but not a plasminogen activator which degrades fibrin by activating plasminogen. SW-1 was found also to degrade fibrinogen. In vivo, the thrombolytic effect of SW-1(4000 u·kg^-1) on venous thrombus in rat was most remarkable versus that of control(P<0.001), but was not divergent versus that of urokinase at the same dose(P>0.05). SW-1 was shown to enhance plasminogen and plasmin activity but did not show significant effect on endogenous t-PA or α₂-PI. SW-1 also caused decrease of fibrinogen level in plasma, indicating that it degrades the fibrin in thrombus and at the same time degrades the circulating fibrinogen to certain extent. The changes of these biochemical data demonstrated the thrombolytic properties of SW-1 in vivo.