Plasminogen, precursor of the active protease plasmin, is a single chain glycoprotein of 92 kDa. Found in plasma at a concentration of 200 ug/ml, it contains 5 disulfide-bonded structures termed “kringles” and a serine protease domain at the carboxy-terminus. Plasmin is primarily responsible for digesting fibrin clots. Angiostatin, a 38 kDa fragment of plasma consisting of kringles 1 through 4, has anti-angiogenisis activity.