1. A crystalline enzyme capable of digesting yeast nucleic acid has been isolated from fresh beef pancreas. 2. The enzyme called "ribonuclease" is a soluble protein of albumin type. Its molecular weight is about 15,000. Its isoelectric point is in the region of pH 8.0. 3. Ribonuclease splits yeast nucleic acid into fragments small enough to diffuse readily through collodion or cellophane membranes. 4. The split products of digestion, unlike the undigested yeast nucleic acid, are not precipitable with glacial acetic acid or dilute hydrochloric acid. 5. The digestion of yeast nucleic acid is accompanied by a gradual formation of free acid groups without any significant liberation of free phosphoric acid. 6. Ribonuclease is stable over a wide range of pH even when heated for a short time at 100°C. Its maximum stability is in the range of pH 2.0 to 4.5. 7. Denaturation of the protein of ribonuclease by heat or alkali, or digestion of the protein by pepsin, causes a corresponding percentage loss in the enzymatic activity of the material.