Materials and Methods.-Bovine pancreatic ribonuclease (RNase) was purchased from the Sigma Chemical Company, St. Louis, Missouri. Test samples of the lot employed in these studies (# R60B-204," chromatographic grade") were subjected, beforeuse, to chromatography4 on the cation exchanger, carboxymethylcellulose3 (Brown Paper Company, Lot 1069; 0.6 meq/gm) and on IRC-50, 6 and were found to consist almost entirely of the major" A" peak with a smaller peak (approximately 5% of the major peak) running in the position of the usual" B" component Amino acid analysis according to the procedure of Spackman, Stein and Moore7 yielded values in good agreement with those reported for the commercial preparations thathave been used for most of the structural studies on this enzyme.