Monocytes cultured in vitro differentiate to a macrophage-like phenotype and undergo functional changes, including reduced capacity for release of TNF-alpha and the soluble p55 receptor for TNF (sTNF-R55) but enhanced capacity for release of the soluble p75 receptor (sTNF-R75). The cytokines IL-4 and IL-10 act on monocytes to suppress the release of pro-inflammatory cytokines, including TNF-alpha, and to influence the release of sTNF-R. We therefore investigated the influence of differentiation over 15 days in vitro on the spontaneous and LPS- and IFN-gamma-induced release of TNF-alpha and sTNF-R from human monocytes and examined the actions of IL-4 and IL-10 on these. Unstimulated monocytes did not release TNF-alpha at any stage but released progressively larger amounts of sTNF-R75 with time. LPS-stimulated release of TNF-alpha declined substantially after the first day and was consistently suppressed by IL-10 and IL-4 but increased by IFN-gamma. Monocytes cultured with IL-10 released more sTNF-R75 at all times and expressed more mRNA for TNF-R75 at day 8. LPS stimulation consistently enhanced both spontaneous and IL-10-augmented release of sTNF-R75, whilst IFN-gamma co-stimulation consistently suppressed them. The influence of IL-4 on sTNF-R75 release, however, depended qualitatively on both the length of time in culture and on conditions of stimulation. The effects of LPS and IFN-gamma on TNF-alpha and sTNF-R75 release were progressively lost with increasing time in culture in the presence of IL-4. sTNF-R55 was not detectable after the first day of culture under any of these conditions. IL-4, IL-10 and IFN-gamma therefore have distinct, but interacting, effects on the balance between TNF-alpha and sTNF-R75 release by maturing monocytes. These interactions may be relevant to the pathogenesis or treatment of TNF-alpha-mediated diseases, where sTNF-R may act to neutralize or stabilise TNF, thereby modifying biological activity.