The expression of the chloramphenicol-inducible chloramphenicol acetyltransferase gene (cat) of the staphylococcal plasmid pUB112 is regulated at the post-transcriptional level. Previous in vivo analyses suggested that the antibiotic stalls ribosomes that are translating a regulatory leader peptide, and that a stalled ribosome activates the ribosome binding site of the acetyltransferase encoding sequence by opening an attenuating leader mRNA hairpin structure. To test this model, we used a Bacillus subtilis S-30 extract for an in vitro translation system and in vitro synthesized cat in RNAs. We showed that the leader portion of the cat transcript acts as a translational attenuator of cat gene expression in absence of chloramphenicol. The drug stimulates acetyltransferase synthesis by a leader mRNA-dependent activation of translation of the cat message. By using 5' end-labeled transcripts and employing the endogenous RNase activity of the S-30 extract we demonstrated that this activation is due to an antibiotic-induced stalling of a ribosome on cat leader mRNA.