A simple quantitative assay method for ribonuclease activity has been developed. This method is based on the decrease of fluorescence intensity emitted by the ethidium bromide bound to RNA due to the degradation of RNA by ribonuclease. The substrate RNA was reacted with ribonuclease A and the fluorescence intensity was measured after the addition of ethidium bromide. The intensity difference was calculated using a blank reaction mixture containing no RNase. Whole cellular RNA substrate produced a significant error and was not suitable for this assay method possibly because of local microheterogeniety caused by high molecular weight rRNA. but satisfying results were obtained with tRNA substrate. The intensity difference increased linearly by raising enzyme concentration up to $2{ imes}10^{-4}$ Kunitz Units of ribonuclease A. More refined and reliable results were obtained by use of initial reaction velocities which were calculated from the plots of intensity difference vs time. A linear relationship between initial velocities and enzyme concentrations was observed up to 0.01 Kunitz Units of enzyme.