Acid RNAase, alkaline RNAase, and alkaline RNAase inhibitor appear to be widespread in higher animals. The livers of all mammalian species tested including that ofa monotreme contained inhibitor active against bovine pancreatic RNAase. Chick liver contained inhibitor active against "activated" chick liver supernatant RNAase, but not against bovine pancreatic RNAase. The rat liver inhibitor showed partial activity against chick liver supernatant RNAase.