Abstract IFN regulatory factor-1 (IRF-1) is a critical effector molecule in IFN signaling and acts as a tumor suppressor and tumor susceptibility gene. IL-12 is a key factor in the induction of innate resistance and generation of Th1 cells and CTL. Our recent study has revealed an intimate relationship between IRF-1 and IL-12 in that IRF-1 regulates the production of IL-12 by selectively controlling transcriptional activation of IL-12 p35 gene. In this work, we find that IRF-1-deficient mice are highly susceptible to N-methyl-N-nitrosourea (MNU)-induced T lymphomas. This susceptibility is associated with strong defects in the expression of IL-12, lymphotoxin (LT)β, and IFN-γ. Consistently, IL-12 p35−/−, IFN-γ−/−, and LTβ−/− mice are also highly vulnerable to MNU-induced carcinogenesis. Administration of rIL-12 to IRF-1−/− mice restores normal expression of LTβ and IFN-γ, and significantly enhances the ability of IRF-1−/− mice to resist MNU-induced pathogenesis. This strongly suggests an IRF-1/IL-12/IFN-γ regulatory axis in tumor surveillance. By DNA microarray analysis, we comprehensively identify differences and patterns in gene expression in splenocytes of wild-type (WT) vs IRF-1−/− mice challenged with MNU. This study contributes to efforts to elucidate the cellular/molecular mechanisms and the downstream players involved in IRF-1-mediated host defense against lymphoproliferative malignancies.