Mouse 3T3 cells, in stationary phase because of serum deprivation, have only half the ribosome content of growing cells. Furthermore, the proportion of protein synthesis devoted to ribosomal proteins is only half that in growing cells. On addition of serum the synthesis of each ribosomal protein increases threefold, demonstrating the coordination of the synthesis of the ribosomal proteins. Half that increase is due to a general increase in total protein synthesis; half is due to a differential increase in ribosomal protein synthesis. The latter is abolished by a concentration of actinomycin D which blocks only ribosomal RNA transcription. The results are discussed with reference to a general hypothesis of growth regulation proposed by Stanners et al. (1979).