Active plasmin, available plasmin, and total plasminogen were measured by Enzo-diffusion fibirn plate techniques in 11 cases and level of tissue activator and tissue fibrinolytic activities in another 11 cases with chronic subdural hematoma. The values were too small to be measured in some instances. Anti-plasmin in the hematoma was less than in the blood plasma. The outer membrane contained about three times more tissue activator than the dura mater, although the inner membrane contained none. Increased tissue activator, which exudes from the extremely vascular outer membrane, transforms plasminogen into plasmin in subdural hematoma, so that plasmin breaks down fibrin and fibrinogen and induces continuous hemorrhage.