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Interferon-γ-inducing factor (IGIF) is a novel cytokine that stimulates T-cell proliferation, augments natural killer (NK) cell lytic activity, and induces interferon- γ (IFN-γ) production in established type 1 T-helper (Thl) cells in the presence of anti-CD3 antibody. The in vitro induction of IFN-γ by recombinant murine IGIF in these cells was more potent than that induced by murine interleukin-12 (IL-12) and occurred apparently independent of murine IL-12. Here we report that subcutaneous injection into mice of tumor cells transfected with murine IGIF complementary DNA (cDNA) resulted in ≥ 10-fold increase of mitogen-stimulated IFN-γ production in cultured splenocytes. In addition, IGIF-transfected Renca and K1735 tumor cells can be rejected in vivo. The IGIF antitumor effect was abrogated in mice that were sublethally irradiated or depleted of both CD4+ and CD8+ T cells but not in mice depleted of either subpopulation alone. The antitumor effect mediated by IGIF appears to be dependent on IFN-γ production, because in vivo neutralization of IFN-γ was accompanied by growth of IGIF-transfected tumors in 100% of the animals. Taken together, our results show that murine IGIF can elicit T-cell-dependent antitumor immunity associated with IFN-γ induction.