Abstract IFN-γ is a key cytokine controlling Brucella infection, and the diverse functions of this cytokine are mediated by IFN regulatory factors (IRFs) such as IRF-1, IRF-2, and IFN consensus sequence binding protein (ICSBP). However, the roles of these three IRFs in Brucella infection have not been investigated. The infection of each IRF-deficient mouse strain provides an opportunity to determine not only the significance of each IRF molecule but also the crucial immune components necessary for host defense during in vivo infection, because respective IRF-deficient mouse strains contain unique immunodeficient phenotypes. Brucella abortus S2308-infected IRF-1−/− mice were dead within 2 wk postinfection, while IRF-2−/− mice contained less splenic Brucella CFU than wild-type mice at the early stage of infection. Infected ICSBP−/− mice maintained a plateau of splenic Brucella CFU throughout the infection. Additional infection of IL-12p40-, NO synthase 2-, and gp91phox-deficient mice indicates that these immune components are crucial for Brucella immunity and may contribute to the susceptibility of IRF-1−/− and ICSBP−/− mice. Immunologic and histopathological analyses of infected IRF-1−/− mice indicate that the absence of IL-12p40 induction and serious hepatic damage are involved in the death of IRF-1−/− mice. These results indicate that 1) IRF-1 and ICSBP are essential transcriptional factors for IFN-γ-mediated protection against Brucella; 2) IL-12, reactive nitrogen intermediates, and reactive oxygen intermediates are crucial immune components against Brucella, and their absence may contribute to the susceptibility of IRF-1−/− and ICSBP−/− mice; and 3) hepatic damage caused by Brucella virulence contributes to the death of IRF-1−/− mice.