PURPOSE: A ribonuclease, obtained from the constricted part of an avian oviduct, and capable of discriminating the keto-base in ribonucleic acid preferentially, cleaving the 5'-phosphate linkage, producing an oligomer having the 2',3'- cyclic phosphoric acid group at the terminal 3'-position.

CONSTITUTION: A novel ribonuclease, obtained from the constricted part of an avian oviduct by the separation and purification with the combination of the column chromatography, gel filtration, electrophoresis, etc., capable of discriminating the keto-base of ribonucleic acid, cleaving the 5'-phosphate linkage preferentially, and producing an oligomer having the 2',3'-cyclic phosphoric acid group at the terminal 3'-position, having almost uniform and about 34,000 molecular weight by the gel filtration with "Sephadex G-100 " and optimal 4.5pH, and the remarkably increasing the activity thereof in the presence of sodium chloride or Mg++ ions in a suitable concentration with the specificity for bases.