Methods of assay for acid and alkaline ribonuclease, for inactive ribonuclease and for ribonuclease-inhibitor have been studied, with special reference to their application to extracts of tissues such as rat-liver. The purity of the RNA used as substrate was found to be a factor of major importance. As a result of metal contaminants, some RNA preparations inactivated the inhibitor and caused a concomitant release of otherwise inactive alkaline ribonuclease. Treatment of these RNA preparations with the chelating agent EDTA enabled satisfactory assays to be made. Results were also obtained which demonstrate the importance of ensuring complete disruption of sub-cellular granules, if full ribonuclease activity is to be measured. A rapid method of purification of the ribonuclease-inhibitor of rat-liver was devised, in order to overcome problems associated with the instability of the inhibitor. Three steps were involved: (a) the preparation of a supernatant fraction; (b) chromatography on a DEAE-cellulose column; (c) chromatography on a calcium phosphate column. A 6000-fold purification was achieved, with a recovery of 20%. The product was stable to storage at 0–4°.