The role of the ribosomal protein S12 (streptomycin protein) in ribosome function and in other metabolic processes in the cell has been investigated. A spontaneous streptomycin-resistant strain of Escherichia coli (SM3) carrying a mutation in the rpsL gene is deficient in its ability to induce the synthesis of the enzyme bets-galactosidase. It was demonstrated that the reduced rate of enzyme synthesis results from deficiencies in both the transcription of the lactose operon and translation of the lactose operon mRNA. The transcription deficiency was in part due to increased catabolite repression and could therefore be partially suppressed by the addition of cyclic AMP. Streptomycin also appeared to partially suppress catabolite repression. In the SM3 mutant strain, the translation of the lactose operon mRNA was only about 60% as efficient as in the parental control, and addition of streptomycin did not alter the translation efficiency. In contrast, both transcription and translation of ribosomal protein mRNA were equally efficient in the two strains. These observations imply that mutational alterations in the ribosomal protein S12 either directly or indirectly alter (i) the extent of catabolite repression, (ii) the efficiency of transcription of the lactose operon even in the absence of catabolite repression, and (iii) the efficiency of translation of some but not all mRNA species in the cell.