Plasminogen is the precursor of plasmin, an active serine protease that dissolves the fibrin of blood clots and acts in many other processes such as embryonic development, tissue remodeling, inflammation and tumor invasion. Synthesized in the kidney, plasminogen is found in plasma and many extracellular fluids. Activated by u- or t-plasminogen activator, the single-chain plasminogen is converted to plasmin that consists of a disulfide bond-linked heavy chain A and light chain B. Heavy chain A contains 5 kringle domains and light chain B corresponds to the serine protease domain. A fragment consisting of the first 4 kringle domains has been named angiostatin, a novel angiogenesis inhibitor.