Activation of immature CD83− dendritic cells (DC) in peripheral tissues induces their maturation and migration to lymph nodes. Activated DC become potent stimulators of Th cells and efficient inducers of Th1-and Th2-type cytokine production. This study analyzes the ability of human monocyte-derived CD1a+ DC at different stages of IL-1β and TNF-α-induced maturation to produce the major Th1-driving factor IL-12. DC at the early stages of maturation (2 and 4 h) produced elevated amounts of IL-12 p70 during interaction with CD40 ligand-bearing Th cells or, after stimulation with the T cell-replacing factors, soluble CD40 ligand and IFN-γ. The ability to produce IL-12 was strongly down-regulated at later time points, 12 h after the induction of DC maturation, and in fully mature CD83+ cells, at 48 h. In contrast, the ability of mature DC to produce IL-6 was preserved or even enhanced, indicating their intact responsiveness to CD40 triggering. A reduced IL-12-producing capacity of mature DC resulted mainly from their impaired responsiveness to IFN-γ, a cofactor in CD40-induced IL-12 p70 production. This correlated with reduced expression of IFN-γR (CD119) by mature DC. In addition, while immature DC produced IL-12 and IL-6 after stimulation with LPS or Staphylococcus aureusCowan I strain, mature DC became unresponsive to these bacterial stimuli. Together with the previously described ability of IL-10 and PGE 2 to stably down-regulate the ability to produce IL-12 in maturing, but not in fully mature, DC, the current data indicate a general resistance of mature DC to IL-12-modulating factors.