Both interleukin-12 (IL-12) and tumor necrosis factor alpha (TNF-alpha) are produced early in intracellular bacterial infection. Depletion of either IL-12 or TNF-alpha by a single injection of specific antibody 4 h before the injection of Brucella abortus 19 led to the exacerbation of infection 2 weeks later. Whereas the effect of IL-12 depletion on resistance was persistent and exacerbation was still significant 6 weeks later, the bacterial numbers in mice depleted of TNF-alpha were similar to the bacterial numbers in control infected mice by 6 weeks postinfection. Massive splenomegaly, which is often seen in 2-week Brucella-infected mice, was not observed in IL-12- or TNF-alpha-depleted mice. Both IL-12- and TNF-alpha-depleted mice showed reduced cell accumulation in the spleen compared with the massive cell accumulation in control infected mice. Granuloma formation in livers was much reduced in IL-12-depleted mice but not in TNF-alpha-depleted mice. Gamma interferon (IFN-gamma) production by cells from TNF-alpha-depleted mice was not significantly different from that of cells from control infected mice. In contrast, the production of IFN-gamma by both CD4+ and CD8+ T cells from IL-12-depleted mice was greatly reduced, compared with that from control infected mice. This effect was still observed when the antibody injection was delayed for up to 7 days postinfection, but injections of anti-IL-12 antibody into mice with established Brucella infection had no significant effect on IFN-gamma production by T cells. Taken together, these results suggested that IL-12 contributed to resistance mainly via an IFN-gamma-dependent pathway and had a profound effect on the induction of acquired cellular resistance. In contrast, TNF-alpha was involved in resistance possibly via direct action on effector cells and may not be essential for the induction of acquired cellular resistance.