1. A comparison has been made between the ribonuclease activities of untreated ribosomes from Escherichia coli B and Pseudomonas fluorescens and the activities of ribosomes on to which ribosomal ribonuclease from E. coli B has been adsorbed. 2. The normal ribosomes from both species were stable in 5-10mm-Mg(2+) (I0.16) at pH6. The RNA in ribosomes from Ps. fluorescens was attacked by the adsorbed ribonuclease under these conditions, whereas the ribosomes from E. coli B were able to adsorb and inhibit this enzyme. 3. Inhibition was also observed with ribosomes from Aerobacter aerogenes, Proteus vulgaris and two other strains of E. coli. It was not observed in ribosomes from three species of Pseudomonas. 4. The inhibition depended on the integrity of the ribosomes and was not observed under conditions of low Mg(2+) concentration that cause irreversible degradation into more slowly sedimenting particles.