To determine whether the production and secretion of TNF and IL-1 by human mononuclear phagocytes could be independently modulated, we examined secretion of TNF and IL-1 by fresh monocytes and monocytes pretreated with IFN-gamma or granulocyte macrophage CSF before LPS stimulation. TNF and IL-1 secretion were in part differentially modulated. Fresh monocytes secreted large amounts of TNF and IL-1 after LPS stimulation and less than 6% as much without LPS. The capacity to secrete TNF in response to LPS decreased slightly in cultured monocytes but was markedly augmented by IFN-gamma (approximately five-fold more than fresh monocytes). In contrast, cultured monocytes secreted less than 5% as much IL-1 as fresh monocytes and, although augmented by IFN-gamma, IL-1 secretion remained much less than by fresh monocytes. These differences in modulation were reflected by differences in the molecular mechanisms regulating TNF and IL-1 secretion. TNF secretion was regulated primarily by changes in the duration of increased transcription and by an apparent increase in translation or protein stability in response to LPS; greater than 95% TNF produced was secreted under all conditions. In contrast, the changes in IL-1 secretion reflected primarily post-transcriptional regulation of IL1-alpha mRNA, transcriptional and post-transcriptional regulation of IL-1 beta mRNA and a decrease in the fraction of IL-1 secreted by cultured compared with fresh monocytes (10 and 60%, respectively). Changes in translational efficiency or protein processing or stability appeared not to be important mechanisms regulating IL-1 secretion. Additional evidence that TNF and IL-1 can be differentially modulated was the selective decrease in TNF secretion and the failure of IFN-gamma to enhance TNF secretion by cultured monocytes from neonates, whereas results for IL-1 were similar with adult and neonatal monocytes. Results with tissue macrophages were similar to those with cultured monocytes. These results indicate that TNF and IL-1 production and secretion by mononuclear phagocytes can be differentially modulated, reflecting in part different mechanisms of regulation; this may allow them to play partially independent roles in the host immune response.