Angiogenin, a potent blood vessel inducing protein, was previously isolated from medium conditioned by a human adenocarcinoma cell line [Fett, J. W., Strydom, D.J., Lobb, R.R., Alderman, E.M., Bethune, J.L., Riordan, J.F., & Vallee, B.L. (1985) Biochemistry 24, 5480-5486]. We now report that a protein which is physically and functionally identical with angiogenin is present in normal human plasma and can be purified to homogeneity by CM 52 and Mono S cation-exchange chromatography. The plasma-derived angiogenin exhibits the same angiogenic and ribonucleolytic activities, amino acid composition, molecular weight, immunoreactivity, and chromatographic behavior as the tumor cell derived protein. Peptide mapping and sequencing studies indicate chemical identity of the two proteins. The present yield of angiogenin from either plasma or serum is 60-150 micrograms/L. These findings demonstrate that angiogenin is not a tumor-specific product and provide further opportunities for the investigation of the role and mechanism of action of angiogenin and its potential diagnostic or prognostic utility.