Vascular plasminogen activator, probably synthesized in the endothelial cells of blood vessel walls, is released into the blood following many types of stimuli. Activator generates plasmin, believed to help maintain blood vessels free of thrombi. Vascular activator displays a very high specific activity, binds fibrin, and is not readily inhibited by any plasma inhibitors, in contrast to the other reported activators, kallikrein, factors XIIa and Xa, which are weak and rapidly neutralized by plasma inhibitors. Purification to homogeneity has been reported starting from pig heart and a perfusate of human cadavers. Little functional characterization has been reported except that activator binds to fibrin and appears to function more rapidly in the presence of fibrin. We wish to further refine our procedure for recovery of released activator from blood, examine whether the reported activators are perhaps degraded forms of a native proactivator in tissue, and pursue functional characterization of the activator and proactivator (if the latter exists and can be isolated). Binding of activator to itself (aggregation), fibrin, fibrinogen, plasminogen, cold soluble globulin and other blood components would be evaluated by sedimentation equilibrium centrifugation in the airfuge, and compared to urokinase in the same situations. The apparent fibrin cofactor effect would be further examined using a chromogenic assay for plasmin and proteolytic fragments of fibrin examined similarly. The goal would be in vitro characterization of the control mechanisms modulating this system to aid in eventual evaluation of its presumed in vivo function in preventing thrombosis.