An acid ribonuclease was partially purified from an acetone powder of porcine brain. This enzyme was an acidic protein with a molecular weight of aroung 70,000. It acted on yeast RNA optimally at about pH 5.9, yielding only a mixture of 3'-mononucleotides, and therefore appears to be an exonuclease. It did not hydrolyze heat-denatured calf thymus DNA or bis(rho-nitrophenyl) phosphate. It was fairly unstable to heat and acid. 2. An alkaline ribonuclease was partially purified from the same source simultaneously. This enzyme was a basic protein with a molecular weight of 25,000-26,000. It was a pyrimidine-specific endoribonuclease, and acted on yeast RNA optimmally at around pH 7.4. It did not hydrolyze heat-denatured calf thymus DNA or bis(rho-nitrophenyl) phosphate. It was fairly stable to heat and acid.
