Purification of plasminogen from pig heart leads to diminished activity measured by its ability to activate purified plasminogen, relative to activity measured by plasma clot lysis assay. This discrepancy is due to the removal of an accessory protein during the purification process. The accessory protein has been isolated in crude form. Many questions are as yet unanswered: (a) is the purified activator in the precursor form which must be activated to its active form for plasminogen activation to occur?, (b) can the precursor form be converted to its active form by proteolytic enzymes, as has been found for urokinase?, (c) is the accessory factor involved in the proactivator to activator conversion?, (d) is the accessory factor or activator destroyed by plasmin generated in the plasminogen activation system to account for the cessation of activation after the initial rapid acceleratory phase?, (e) what are the best methods for purifying accessory factor so that its physico-chemical characteristics and role in plasminogen activation can be elucidated? These are some of the areas that will be investigated. In addition, isolation of activator from human tissues and preparation of an antiserum against it is an objective in the coming year.