Experimental autoimmune encephalomyelitis (EAE) is mediated by myelin-specific CD4+ T helper 1 (Th1) cells, while recovery from the disease is associated with the presence of Th2 cells. Here we used animals with targeted deletion of the T-bet gene to determine its role in the progression of EAE. T-bet regulates the production of interferon-gamma (IFN-gamma) in CD4+ and natural killer cells, and CD4+ T cells from T-bet-deficient mice were unable to differentiate into a Th1 phenotype. Moreover BALB/c mice deficient in T-bet were resistant to the induction of EAE disease, with minimal inflammatory infiltrates in the central nervous system. These mice were resistant to EAE induction even when PLP(180-199) peptide specific effector T cells from BALB/c wild type were transferred to BALB/c T-bet-deficient mice. This resistance to EAE is may be caused by the production of the anti-inflammatory cytokine interleukin-10 (IL-10) from the spleen cells upon ex vivo stimulation with PLP(180-199) peptide and in vivo presence in the central nervous system. There was no difference in the recall responses in spleen cells from T-bet-deficient and wild type mice; however, less secretion of IFN-gamma was observed from primed splenocytes. The expression of IFN-gamma was less in the central nervous system of T-bet-deficient mice whereas IL-10 was significantly higher in T-bet-deficient as compared to wild type mice. These data indicate that T-bet genes play a critical role in maintaining the encephalitogenic nature of CD4+ T cells in autoimmune responses during EAE disease progression.