A method for separate measurement of the amounts of the native form and partially degraded form of plasminogen and plasmin in human circulating blood was devised based on their different mobilities on disc polyacrylamide gel electrophoresis. The relative amounts in the circulating blood as estimated by the method were as follows: 78.5% native form, 6.1% partially degraded form and 15.5% plasmin, in the resting state of healthy adults. The partially degraded form increased to about 50% after strenuous exercise load. The present study showed that relatively large amounts of the partially degraded form can be produced in the circulating blood, even though abundant plasmin inhibitors exist there. These data suggest that the partially degraded form may play an important role in thrombolysis in vivo.