1. Basic dyes (methyl green/pyronin, and toluidin blue), in conjunction with ribonuclease, may be used to detect the cytochemical localization of ribonucleic acid. This technique has been examined critically. 2. The fixative used is important and results vary with different materials. It is suggested that Zenker and Serra fixation should be compared in preliminary investigations. Unexplained anomalies arise in the use of frozen-dried material. 3. Details are given of the methods of use of ribonuclease and of the dyes, and a general technique is given for preliminary investigations. 4. It is concluded that there is a general parallelism between intensity of staining with basic dyes and content of ribonucleic acid, but there is no evidence yet available to justify the use of such staining for the quantitative estimation of ribonucleic acid.