Tissue activator of the plasmin system is easily extracted into such high molar solutionsas 1-2 M KSCN, KC1, KI or 30% sucrose,but not into physiological saline solution. A question, therefore, arises as to by what mechanism such activator can enter intothe circulation. In order to clarify the activation mechanism of the plasmin system and to determine whether or not the tissue activator can enter into the circulation, fibrinolyticc activity of blood obtained from the vasa efferentia of the uterus was determined and compared with that from the eubital vein. A marked elevation of fibrinolytic activity wasobserved in blood from the vasa efferentia of the uterus. It was also found that the endometrial tissue activator extracted into KSCN solution was completely transferred to th