Type I IFNs (IFNalpha/beta) are approved for the treatment of a variety of diseases, including the autoimmune disease multiple sclerosis (MS). The proinflammatory cytokines IL-12 and IFN-gamma have been proposed to contribute to the pathogenesis of MS. Since dendritic cells (DCs) are recognized as major producers of IL-12p70 and promote the development of IFN-gamma-producing Th1 cells, we investigated the direct effect of IFNalpha/beta on monocyte-derived DCs at different stages of development. We demonstrate that IFNalpha/beta enhance IL-12p70 production by immature DCs but inhibit IL-12p70 production by mature DCs. Importantly, IFNalpha/beta strongly counteracted the IL-12-enhancing effect of IFN-gamma on DCs irrespective of their maturation status. Exposure of DCs to IFNalpha/beta during maturation does not affect their maturation or cytokine profile upon CD40 ligation. The differential modulatory effect of IFNalpha/beta on the IL-12-producing capacity of DCs and their cross-regulatory effect on IFN-gamma may reduce inflammatory processes and therefore be therapeutically effective in MS.