An effective Th1 type cell-mediated immune response against cancer cells is critical in limiting cancer progression. Gadd45b, a signaling molecule highly up-regulated during Th1 type responses, is studied for its role in limiting tumor growth. Mouse B16 melanoma cells implanted into Gadd45b(-/-) mice grew faster than those in WT or Gadd45b(+/-) littermate controls. The defect of Gadd45b(-/-) mice in tumor immunosurveillance was attributed to the reduced expression of IFN-gamma, granzyme B, and CCR5 in Gadd45b(-/-) CD8(+) T cells at the tumor site. Activation of p38 MAP kinase, but not ERK or JNK, by either TCR-stimuli or IL-12 and IL-18 is diminished in Gadd45b(-/-) CD8(+) T cells, resulting in reduced production of IFN-gamma. In addition, mRNA of T-bet and Eomes were reduced in Gadd45b(-/-) CD8(+) T cells, supporting a critical role of Gadd45b in shaping the Th1 fate. More importantly, the tumor vaccination, which is effective in WT mice, failed in Gadd45b/Gadd45g doubly deficient mice. Collectively, these data demonstrate that members of the Gadd45 gene family are important for anti-tumor immune responses.