The homology of angiogenin and pancreatic RNase A provides a compelling reason to systematically compare the characteristics of the two proteins using the chemical modification approaches that proved essential to understanding the action of RNase. Reagents specific for histidine, lysine, and arginine markedly decrease the ribonucleolytic activity of angiogenin, much as has been observed for RNase A. Activity is abolished by reduction of the disulfide bonds and is restored by reoxidation. Methionine, tyrosine, and carboxyl group reagents have no significant effect. From the point of view of reactivity, the histidine and lysine residues in angiogenin are severalfold less susceptible to modification than those in RNase A. Arginine reagents, on the other hand, inactivate angiogenin considerably faster than RNase A. Considering specificity, bromoacetate inactivates angiogenin at pH 5.5 by modifying 1.5 histidines, but lysine and arginine reagents are less specific. Thus, 3.8 and 6.3 residues, respectively, are modified by 1-fluoro-2,4-dinitrobenzene and by formaldehyde plus cyanoborohydride, under conditions where activity decreases by approximately 80% in both cases. With phenylglyoxal, 6.7 arginines are lost when there is 92% inactivation. Poly(G) prevents inactivation by lysine and arginine reagents, and phosphate protects against the effects of lysine modification. Thus, the functional consequences of these modifications likely reflect the loss of critical residues rather than general conformational effects.