Interest has recently been revived concerning the efficacy and safety of crystalline trypsin (Enzar) administered intravenously in the treatment of intravascular thrombosis. This has followed reports by Innerfield, Schwartz, and Angrist1 stating that trypsin intravenously induces lytic effects upon artificially formed intravascular thrombi in rabbits and dogs. They also claim that with certain amounts of trypsin "complete dissolution" of such clots may be obtained "with restoration of circulatory integrity." These observations have led them to administer trypsin intravenously to patients with what they claim to have been gratifying results.2 As the attempted dissolution of an intravascular protein thrombus with a proteolytic enzyme would appear to be an empiric but rational procedure, further consideration and investigation seemed mandatory. The concomitant possibility that it might activate the profibrinolysin system of the plasma had also to be considered. Reference to the literature, however, revealed that in certain amounts trypsin will, in