A pair of ribonuclease assays have been developed which offer improvements in specificity, simplicity, and/or sensitivity over current procedures. The assays measure the rate of adenosine release upon ribonuclease hydrolysis of 3'-adenosyl dinucleoside monophosphate substrates. Adenosine formation is spectrophotometrically determined by combining a coupled-enzyme system (adenosine deaminase or an adenosine deaminase/nucleoside phosphorylase/xanthine oxidase combination) to the ribonuclease cleavage. As demonstrated by a brief characterization of the ribonuclease activities in several mouse tissues, the methods demonstrate the advantage of being able to discriminate between ribonucleases of differing substrate specificities. An interesting guanosyl(3'-5')adenosine-specific ribonuclease in mouse brain has been identified using these assay methods.