Angiostatin, a 38 kDa internal fragment of plasminogen, is an antiangiogenic endothelial cell inhibitor. It regresses several primary and metastatic tumors in mice. To produce recombinant angiostatin for further structural and functional studies, the mouse angiostatin gene preceded by a sequence including a signal peptide of plasminogen was introduced into baculovirus. Recombinant murine angiostatin was purified from the culture medium of angiostatin baculovirus-infected insect cells (yield = 1 mg/liter) with a single-step of lysine-Sepharose chromatography. The angiostatin baculovirus-infected insect cells expressed and secreted a 52 kDa polypeptide that demonstrated all of the biological activities of angiostatin. A partial amino acid sequence of the NH2-terminus of the secreted protein revealed that the signal peptide was recognized and properly cleaved in insect cells. The recombinant murine angiostatin potently inhibited the proliferation of bovine capillary endothelial cells in vitro (half maximal inhibition = 50 ng/ml) and suppressed the growth of primary Lewis lung carcinoma in vivo (6 mg/kg/day, T/C = 0.08).