Site-specific mutagenesis of the blood vessel-inducing protein angiogenin has been used to further explore both its homology to pancreatic ribonuclease and the functional roles of particular residues. Replacement of Asp-116 in angiogenin by either asparagine (D116N), alanine (D116A), or histidine (D116H) markedly enhances both its ribonucleolytic activity and angiogenic potency. Activity toward tRNA is 8-, 15-, and 18-fold greater than native angiogenin for D116N-, D116A-, and D116H-angiogenin, respectively. The enzymatic specificity of angiogenin, however, has been maintained. Thus, cleavage of 18S and 28S rRNA by the most active His-116 mutant yields the same pattern of polynucleotide products as from angiogenin, whereas there are only minor alterations in activity with cytidylyl(3',5')adenosine and uridylyl(3',5')-adenosine. Extensive biological assays on the chicken embryo chorioallantoic membrane demonstrate that D116H-angiogenin is one to two orders of magnitude more potent in inducing neovascularization than native angiogenin, which correlates well with enhanced enzymatic action. These results support the proposition that the enzymatic and angiogenic activities on angiogenin are interrelated.