IRF‐8/ICSBP and IRF‐1 are IRF family members whose expression is induced in response to IFN‐γ in macrophages. IL‐12 is a cytokine produced in macrophages that plays a critical role in host defense. IFN‐γ and bacterial lipopolysaccharide (LPS) induce IL‐12p40 transcription, which is necessary for the production of IL‐12. We have previously shown that IL‐12p40 expression is impaired in ICSBP‐deficient mice and that transfection of ICSBP together with IRF‐1 can activate IL‐12p40 expression in mouse macrophage cells. To further study the role of ICSBP and IRF‐1, we investigated murine IL‐12p40 promoter activity in the macrophage cell line RAW 264.7. We show here that co‐transfection of ICSBP and IRF‐1 synergistically stimulates IL‐12 promoter activity to a level comparable to that induced by IFN‐γ/LPS. Mutation of the Ets or NFκB site previously shown to be important for IL‐12p40 transcription did not abolish the activation by ICSBP and IRF‐1. However, mutation of the ISRE‐like site found downstream from the NFκB and C/EBP sites abrogated the activation by ICSBP and IRF‐1. Together, these results indicate that ICSBP and IRF‐1 cooperatively stimulate murine IL‐12 transcription through a novel regulatory element in the murine promoter.