Angiostatin is a 38 ku circulating angiogenesis inhibitor purified from the serum and urine of mice bearing a murine Lewis lung carcinoma. It is regarded as an internal fragment of plasminogen containing the first 4 Kringle structures. In vitro,angiostatin specifically inhibits endothelial cell proliferation. In vivo,it inhibits angiogenesis and suppresses the growth of such primary tumors as Lewis lung carcinoma and hemangioendotheliolma in mice.