Human menstrual discharge contained an activator of plasminogen, a fibrinolytic enzyme (plasmin), large amounts of a precursor of a plasminogen activator and no plasminogen. The activator of plasminogen in menstrual discharge was similar to the plasminogen activator present in the human endometrium and in other tissues. It differed in stability from the plasminogen activator formed in blood by addition of streptokinase. The presence of this activator is believed to explain the absence of fibrin in normal menstrual discharge and to be the cause of the fluidity of menstrual blood.