1.A ribonuclease, (RNase M) [EC 2.7.7.17 ribonucleate nucleotido-2’-transferase (cyclizing)] was isolated from “Molsin” (Aspergillus saitoi). RNase M was purified about 125 fold by means of column chromato-graphy using IRC-50, phospho-cellulose, DEAE-cellulose and Sephadex G-75. 2. RNase M is very similar in molecular weight to RNase T2, from Asp. oryzae. 3. RNase M has a base specificity similar to that of RNase T2, and releases adenylic acid more rapidly than the other nucleotides. However, RNase M is different from RNase T2 in the more precise base specificity. 4. RNase M is different from RNase T2 in the pH optimum as examined with RNA and U-cyclic-p as substrates. 5. The enzyme is inhibited markedly by Cu++, Zn++, Hg++ and Cd++ and less stable against heat treatment than RNase A or T1