The use of interleukin-18 (IL-18) together with IL-12 induced high levels of IFN-gamma in tumor-bearing mice and regression of liver tumors that was abolished in IFN-gamma((-/-)) mice. Natural killer (NK) and NKT cells were the major producers of IFN-gamma in the livers of mice treated with IL-18 and/or IL-12. Liver NK cells were significantly increased by treatment with IL-18/IL-12, whereas the degree of liver NKT cell TCR detection was diminished by this treatment. Reduction of NK cells with anti-asGM1 decreased the antitumor activity of IL-18/IL-12 therapy and revealed NK cells to be an important component for tumor regression in the liver. In contrast, the antitumor effects of both IL-18 and IL-12 were further increased in CD1d((-/-)) mice, which lack NKT cells. Our data, therefore, show that the antitumor activity induced in mice by IL-18/IL-12 is NK and IFN-gamma dependent and is able to overcome an endogenous immunosuppressive effect of NKT cells in the liver microenvironment. These results suggest that immunotherapeutic approaches that enhance NK cell function while eliminating or altering NKT cells could be effective in the treatment of cancer in the liver.