In Escherichia coli the genes coding for the 52 ribosomal proteins (r-proteins) are organized into a number of transcription units located at various regions on the bacterial genome. The expression of r-protein genes is balanced so that individual r-protein synthesis rates change coordinately in response to changing environmental conditions, and significant amounts of free r-proteins do not exist in the cellular pool. We have suggested a model for the balanced regulation of r-protein gene expression, namely that r-protein synthesis and ribosome assembly are coupled so that r-proteins not incorporated into ribosomes prevent the further translation of r-protein mRNA by a feedback regulatory mechanism. The model was tested in vitro by examining the effect of purified r-proteins on DNA directed r-protein synthesis, and in vivo by examining the effect of overproduction of certain r-proteins on the synthesis rates of other r-proteins. In vitro experiments have revealed that some r-proteins (L1, L4, L10, S4 and S8) can selectively inhibit the synthesis of r-proteins whose genes are in the same operon as their own, and that this specific feedback regulation occurs at the level of translation rather than at the level of transcription of mRNA. Regulatory roles for L1, S4 and L4 have also been established by in vivo experiments. We have studied further the feedback regulatory properties of S8 in vivo and in vitro, and report here that the protein regulates a part of the spc operon.