The amplification of cytokine mRNA following incubation of macrophages with inflammatory stimuli and protein synthesis inhibitors has been related to stabilization of labile mRNA species containing the 3'AUUUA consensus sequence. In the present study, cycloheximide-treated lipopolysaccharide (LPS)-stimulated murine peritoneal macrophages had a five- to six-fold increase in tumour necrosis factor (TNF) mRNA when compared to parallel LPS-stimulated controls. Interleukin-1 beta (IL-1 beta) mRNA levels in these cells, however, were significantly lower than the LPS controls. The down-regulation of IL-1 beta by cycloheximide was not apparent for IL-1 alpha mRNA, which had a two- to three-fold increase in the LPS-stimulated cycloheximide-treated macrophages. A similar profile was observed in vivo in which up-regulation of TNF, but not IL-1 beta mRNA, was apparent in mice administered cycloheximide plus LPS relative to LPS alone. Cycloheximide-treated LPS-stimulated macrophages demonstrated a significant increase in transcriptional activity for TNF, but not IL-1 beta, by nuclear run-on transcription assays and an increase in the amount of the nuclear binding factor NFKB when compared to LPS controls. The cycloheximide-mediated increase in TNF mRNA was also related to an increased stability of the TNF message, while no significant increase in stability was apparent in IL-1 beta mRNA. Therefore, the differential expression of TNF and IL-1 beta mRNA in cycloheximide-treated macrophages involves both transcriptional and post-transcriptional regulatory mechanisms.