Ribonuclease (RNase) activity has been assayed by monitoring the shift in the absorbance maximum of methylene blue upon intercalation into high-molecular-weight RNA. After preincubation of yeast RNA with methylene blue, the initial rate of hydrolysis can be followed spectrophotometrically at a wavelength of 688 nm. This allows enzyme kinetic studies of RNases with their probable native substrate, RNA, in place of the artificial ones, such as dinucleoside phosphates or cyclic monophosphates, currently used.