Center for Biochemical and Biophysical Sciences and Medicine and Departments of Biological Chemistry, Pathology, and Radiology, Harvard Medical School, and Brigham andWomen’s Hospital, Boston, Massachusetts 02115 Received May 9, 1985 abstract: The first human tumor derived protein with in vivo angiogenic activity to be obtained in pure form has been isolated from serum-free supernatants of an established human adenocarcinoma cell line (HT-29) and named angiogenin. It was purified bycation-exchange and reversed-phase high-performance liquid chromatography; the yield was~ 0.5 pg/L of medium. Biological activity of angiogenin was monitored throughout purification by using the chick embryo chorioallantoic membrane assay. Statistical evaluation demonstrates that it displays activity in this system with as little as 35 fmol per egg. Moreover, only 3.5 pmol is required to induce extensive blood vessel growth in the rabbit cornea. The amino acid composition of this basic (isoelectric point> 9.5), single-chain protein of molecular weight~ 14 400 has been determined. The amino terminus is blocked, and the carboxyl-terminal residue is proline.