Native bovine plasminogen is converted to an altered plasminogen and then to plasmin I by either urokinase, plasmin I, or plasmin II. Plasmin I is converted by self-catalysis to plasmin II. Plasmin II degrades itself to the inactive fragment peptide A. In the conversion of native plasminogen to plasmin I, a peptide bond is cleaved giving two chains (designated A1 and B) which are held together by one or more disulfide linkages. When plasmin I is converted to plasmin II, a polypeptide chain is removed from the A1 chain of plasmin I to give plasmin II containing A2 and B chains. When plasmin II is converted to peptide A, most of the B chain is lost. During activation of bovine plasminogen, a molecular entity (termed polypeptide C) with an apparent molecular weight larger than the plasminogen is formed. It is a continuous heterogeneous polypeptide chain.