Abstract: A plasminogen proactivator and a corresponding activator has been isolated from mammalian and avian, especially human, plasma which is characterized within a given species as a single, electrophoretically and immunologically homogeneous protein. The activator acts as a catalyst to initiate fibrinolytic activity in plasma and is therefore useful in controlling clotting which occurs, e.g. in venous thrombosis or arterial occulsion, and in diagnosing conditions which predispose to thromboembolic phenomena. The proactivator has a long useful in vivo half life and can be used to provide a reservoir for maintaining the fibrolytic potential of blood.