The balance between Th1 and Th2 development is determined by IL-4 and IL-12. While the role for CD4+ NK1. 1+ T (NKT) cells in influencing this balance has been recognized based on their capacity to produce IL-4, it is unknown how IL-12 is produced in the innate immune system in which they participate. This study demonstrates that Ag-activated CD4+ NKT cells express CD40 ligand (CD40L)(CD154), which engages CD40 on APC and stimulates them to produce IL-12. Culture of B cell-depleted spleen cells from C57BL/6 mice with α-galactosylceramide (α-GalCer) capable of selectively stimulating Vα14/Jα281+ NKT cells resulted in the production of IL-12 together with IFN-γ and IL-4. α-GalCer-induced IL-12 production occurred in IA b β-deficient mice, but not in β 2-microglobulin-deficient and Vα14/Jα281 TCR-deficient mice, and was inhibited by anti-CD40L mAb. Of CD4+ and CD4− NKT cells, the capacity to express CD40L/CD154 and trigger IL-12 production following α-GalCer stimulation was exhibited preferentially by the CD4+ NKT subset. IL-12 production was also observed in α-GalCer-treated mice. Production of IL-12 preceded IFN-γ production, and IL-12 was required for IFN-γ, but not IL-4, production. A stimulatory/inhibitory relationship existed between IL-12 and IL-4 production. These results illustrate a novel function of CD4+ NKT cells that could be involved in the regulation of Th1 vs Th2 development.