Ribonuclease and deoxyribonuclease were less active towards RNA and DNA, respectively, in the presence of proteins, especially fractionated histones. Basic proteins complex with nucleic acids and stabilize them against digestion by nucleases. Stabilization depends on the nature of the protein; the effect was lost when the protein was acetylated. Basic proteins reduce RNA synthesis in a rat liver nuclei RNA polymerase system, at the same time protect the newly synthesized RNA against ribonuclease action. © 1968.