Disclosed is both a method for preparing a plasminogen and a method for preparing a reversibly inactive acidified plasmin by activating the plasminogen. The prepared plasminogen is typically purified from a fraction obtained in the separation of immunoglobulin from Fraction II+III chromatographic process and eluted at a low pH. The prepared plasmin is isolated and stored with a low pH-buffering capacity agent to provide a substantially stable formulation. The reversibly inactive acidified plasmin may be used in the administration of a thrombolytic therapy.