IFN-α and IL-12 are macrophage-derived cytokines that enhance innate and Th1 immune responses. However, there is little information regarding IFN-α and IL-12 target genes that would be involved in mediating the immunostimulatory effects of these cytokines. The interferon regulatory factor (IRF) family of transcription factors is known to be involved in controlling lymphocyte differentiation and functions. In this work we have studied the effect of IFN-α and IL-12 on the expression of IRF transcription factors in human NK and T cells. Both IFN-α and IL-12 strongly up-regulated IRF-1, IRF-4, and IRF-8 mRNA and protein expression. The binding of IRF-4 and IRF-8 to the λB gene enhancer sequence was also increased following IFN-α- and IL-12-treatment of NK and T cells. A GAS element from the promoter region of the IRF-4 gene was identified. Following stimulation of cells with IFN-α or IL-12, Stat4 was found to bind to this IRF-4 GAS element, as detected by EMSA and DNA affinity binding, implying that the IRF-4 gene is directly activated by both cytokines. Our results suggest that IFN-α and IL-12 may enhance innate and Th1 immune responses by inducing IRF-1, IRF-4, and IRF-8 gene expression.