Interleukin-12 (IL-12) plays a critical role in producing an immune response, as indicated in many ways, e.g., induction of interferon-gamma (IFN-gamma), and augmentation of the cytotoxic activity of resting activated T cells and natural killer (NK) cells. In this study, we examined whether intratumoral injection of a recombinant retrovirus vector expressing IL-12s induce antitumor and antiangiogenic effects in a murine model using a murine head and neck squamous cell carcinoma (NR-S1). In vitro the levels of vascular endothelial growth factor (VEGF) mRNA and protein expression were decreased in IL-12 gene transfected NR-S1 cell. in vivo direct IL-12 gene therapy resulted in significantly remarkable inhibition of tumor growth compared to the control group. The tumor regression by direct IL-12 gene therapy was also associated with decreased vessel density, and apoptosis and increased infiltration of CD8(+) T cells and CD56(+) NK cells in the tumor increased. Also, the number of IFN-gamma expressed cells of spleen cells was increased in the treatment group compared with the control group. These results suggested that direct IL-12 gene therapy appears to be effective in reducing tumor growth by triggering both antiangiogenic effects and an immunological enhancing mechanism through induction of IFN-gamma.