Pancreatic ribonuclease (RNase I) catalyzes cleavage of the phosphodiester bond between the 5"-ribose of a nucleotide and the phosphate group attached to the 3"-ribose of an adjacent pyrimidine nucleotide forming a 2",3"-cyclic phosphate which may then be hydrolyzed to the corresponding 3"-nucleoside phosphate. Ribonuclease A has a molecular weight of 13,700 daltons. It operates in an optimum pH range of 7.0-7.5. Ribonuclease B has a molecular weight of 14,700 ± 300 daltons. It is a glycoprotein which possesses an amino acid composition indistinguishable from that of RNase A. It contains 6 residues of mannose and 2 residues of N-acetylglucosamine per molecule. It is a glycosylated derivative of RNase A. Ribonuclease is inhibited by heavy metal ions and it is competitively inhibited by DNA. Since Molecular Biology Grade RNase A (Product R128619) is essentially free of DNase and protease activities, this product is useful in removing RNA from DNA in nucleic acid work and where other enzymes are used or where intact proteins must be recovered.