1. Plasminogen activator (vascular plasminogen activator) was extracted from the in situvascular trees of human cadavers, and was partially purified by ammonium sulfate fractionation, zinc precipitation, and ethanol fractionation followed by DEAE-cellulose chromatography and Sephadex G-200 gel nitration. The increase in purity of the final activator preparation over that of the original extract was approximately 200-fold. 2. The vascular activator was labile and rapidly lost its activity at pH values above 8 and below 4. The stability of the activator was dependent on the salt concentration of the medium in which the activator was dissolved; higher salt concentrations tended to stabilize the activator. The activator was inactivated by dithiothreitol but was unaffected by p-chloromercuribenzoate. 3. The vascular activator preparation did not possess any proteolytic activity when tested on fibrin, casein, and denatured hemoglobin, but readily activated plasminogen to plasmin [EC 3.4.4.14]. The optimum pH of the activator for plasminogen activation was around 8.5, which is similar to that of urokinase. The activator and urokinase were found to have the same Km value for plasminogen but had different Km values for the synthetic substrate acetylglycyllysine methyl ester. 4. Gamma-globulins prepared from specific antisera to human urokinase did not inhibit the activity of the vascular activator, whereas they strongly inhibited urokinase activity. 5. The molecular weight of the vascular activator was estimated to be in the region of 80,000 by gel filtration. 6. It is suggested on the basis of these findings and other evidence previously reported that vascular activator and urokinase are similar, but not identical, enzymes.