We studied the synthesis of ribosomal proteins encoded by the S10 operon, an eleven gene operon from the str-spc region of the E. coli chromosome, using a lambda fus3 DNA-directed, in vitro protein synthesizing system. Addition of ribosomal protein L4 (1 microM) to in vitro protein synthesis reactions caused selective inhibition of synthesis of the promoter-proximal proteins of the S10 operon, S10, L3, L4, L23 and possibly L2. Proteins of the S10 operon other than L4 did not cause selective inhibition of protein synthesis. Autoregulatory ribosomal proteins previously identified from other operons, L1, S4 and S8, did not inhibit protein synthesis from the S10 operon; nor did L4 cause significant inhibition of protein synthesis from operons other than the S10 operon. As with L1, S4 and S8, L4 inhibits gene expression at the level of translation.