Post-transcriptional controls, including changes in both mRNA translational activity and stability, play an important role in the regulation of ribosomal protein gene expression in developing Dictyostelium discoideum cells. Previously we have shown that the mechanisms which regulate the translational activity of the r-protein mRNAs operate at the level of translational initiation and do not involve changes in polyadenylation or capping. By analysing the translational behavior of chimeric and mutant mRNAs in transformed cells, we have now been able to localize the determinants of translational activity of one of the r-protein mRNAs to the 5'-untranslated region. Although this and other r-protein mRNAs differ strikingly from the Dictyostelium consensus in the region of the initiator AUG codon, we find that improving the match to that consensus does not increase the translational activity of the message in developing cells. Current experiments are designed to determine whether translational regulation is mediated by strong interactions with specific inhibitors or by weak interactions with translational initiation factors.