Abstract 1. 1. Ribonuclease (RNAase) activity in milk of mouse, rat, rabbit, dog, goat, cow and human and in mammary tissue of mouse and rat has been determined at pH 7.4 under various ionic conditions. 2. 2. RNAase in milk of rabbit, cow, goat, human, dog and mouse exhibited no strong ionic preference while activity of rat milk RNAase was greatly enhanced by Ca 2+. Enzyme from every source preferred pyrimidine over purine homopolymers as substrate. 3. 3. Comparing the maximal activity per ml found for each species, milk of rabbit, rat and cow had the highest activity, that of human and goat was intermediate and that of dog and mouse was the lowest. Differences between strains were smaller than interspecific differences. 4. 4. In rat and mouse, milk RNAase activity is a function of mammary RNAase activity in the gland.