Preparations of nucleic acids obtained by extraction of mouse liver, HeLa cells and cell fractions with phenol and deoxycholate have been characterized with regard to the differential solubility of ribonucleic acid and deoxyribonucleic acid in ethanol, density-gradient centrifugation and the presence of high-molecular-weight contaminants. Ribonucleic acid obtained by this method is less soluble than deoxyribonucleic acid. It was precipitable with 20 % ethanol, nearly free of deoxyribonucleic acid, but containing 4-5 times its weight of polysaccharide which is not removed by repeated fractional precipitation nor entirely by ~-amylase (EC 3.2.1.1) digestion, but is removed by density-gradient centrifugation. Deoxyribonucleic acid could be subsequently precipitated with 50 % ethanol free of ribonucleic acid but contaminated with polysaccharide. The buoyant density of the latter is identical with deoxyribonucleic acid and they are not separated by density-gradient centrifugation. The contaminating polysaccharide appears to be a single entity, the fl-subunit of glycogen granules. Its isolation and some of its properties are described. Its effect upon the properties of the nucleic acids is discussed.