Production of cytokines is one of the major mechanisms employed by CD4(+) T cells to coordinate immune responses. Although the molecular mechanisms controlling T cell cytokine production have been extensively studied, the factors that endow T cells with their ability to produce unique sets of cytokines have not been fully characterized. Interferon regulatory factor (IRF)-4 is a lymphoid-restricted member of the interferon regulatory factor family of transcriptional regulators, whose deficiency leads to a profound impairment in the ability of mature CD4(+) T cells to produce cytokines. In these studies, we have investigated the mechanisms employed by IRF-4 to control cytokine synthesis. We demonstrate that stable expression of IRF-4 in Jurkat T cells not only leads to a strong enhancement in the synthesis of interleukin (IL)-2, but also enables these cells to start producing considerable amounts of IL-4, IL-10, and IL-13. Transient transfection assays indicate that IRF-4 can transactivate luciferase reporter constructs driven by either the human IL-2 or the human IL-4 promoter. A detailed analysis of the effects of IRF-4 on the IL-4 promoter reveals that IRF-4 binds to a site adjacent to a functionally important NFAT binding element and that IRF-4 cooperates with NFATc1. These studies thus support the notion that IRF-4 represents one of the lymphoid-specific components that control the ability of T lymphocytes to produce a distinctive array of cytokines.