Abstract: Santer, Melvin (Haverford College, Haverford, Pa.), and Josephine R. Smith . Ribonuclease sensitivity of Escherichia coli ribosomes. J. Bacteriol. 92: 1099–1110. 1966.—The ribonucleic acid (RNA) contained in 70 S ribosomes and in 50 S and 30 S subunits was hydrolyzed by pancreatic ribonuclease. A 7% amount of the RNA was removed from the 70 S particle; at 10 −4 m magnesium concentration, a maximum of 24 and 30% of the RNA in the 50 S and the 30 S fractions, respectively, was removed by ribonuclease. At the two lower magnesium ion concentrations, 50 S ribosomes did not lose any protein, whereas 30 S ribosomes lost protein as a result of ribonuclease treatment. A number of proteins were removed from the 30 S particles by ribonuclease, and these proteins were antigenically related to proteins present in 50 S ribosomes. The differential effect of ribonuclease on 50 S and 30 S ribosomes suggested that they have structural dissimilarities.