We investigated the effects of recombinant C5a (rC5a) on gene expression and synthesis of interleukin-1 beta (IL-1 beta) and tumor necrosis factor (TNF) in fresh human peripheral blood mononuclear cells (PBMC). Total (cell-associated and secreted) cytokine synthesis was measured. In the strict absence of endotoxin (lipopolysaccharide [LPS]), rC5a resulted in a small but statistically insignificant increase in immunoreactive IL-1 beta and TNF, as well as in IL-1 and IL-6 bioactivity. On the other hand, rC5a induced marked transcriptional activation of IL-1 beta and TNF in a dose-dependent fashion with an optimal concentration of 50 ng/mL. The rC5a-induced cytokine messenger RNA (mRNA) was not spontaneously translated into protein. At 50 ng/mL, rC5a induced the same levels of mRNA for IL-1 beta and TNF as 1 ng/mL of LPS, whereas LPS induced 12 times more IL-1 beta protein and 70 times more TNF protein than rC5a alone. The C5a-induced mRNA half-life was the same as that induced by LPS. Formyl-Meth-Leu-Phe (fMLP) did not induce cytokine transcription. Pretreatment with rC5a enhanced cytokine synthesis induced by other stimuli. After 2 hours of preincubation with rC5a, PBMC synthesized 3 to 10 times more IL-1 beta and TNF on subsequent stimulation by LPS or IL-1 itself. We conclude that rC5a provides primarily a transcriptional but not translational signal for IL-1 beta and TNF; the half-life of the untranslated mRNA is the same as that of translated message; rC5a-induced transcription upregulates PBMC for enhanced synthesis of these cytokines; and a translational signal can be provided by LPS or IL-1 itself.