The present data suggest that different enzymes are involved in the metabolic interaction of s-RNA with liver ribosomes and the subsequent peptide-forming step (s-RNA was derived from rat liver, unless indicated otherwise). Association between s-RNA and ribosomes may occur under conditions which exclude polypeptide formation, as was shown in a system lacking enzymes required for peptide synthesis and containing puromycin. s-RNA treated with snake venom diesterase lost its ability to interact with liver microsomes, but this ability could be restored by incubating the s-RNA with cytidine triphosphate, adenosine triphosphate, and pyrophosphorylase. The different behavior of Escherichia coli and rat liver ribosomes, as observed under the conditions of the present experiments, is discussed.