Abstract: To test LLC/mIL- 12, a tumor vaccine for murine Lewis lung carcinom (LLC), and to assess radiosensitization of the murine LLC cells after transfection with the murine IL- 12 gene in Lewis lung tumor- bearing mice. Methods: The recombinant plasmid pcDNA3.1 (+) - mIL- 12 was transfected into the LLC cells via liposome, and then clones positive for LLC/ mIL- 12 were obtained by screening with G418. Hypodermic inoculation of 2×106 LLC cells into the C57BL/6 mice (n =10) was conducted and the mice were randomized into 4 groups after tumorigenesis. The tumor vaccine was administered to the vaccine group and the combination group on day 0, 7 and 14, and local irradiation with 2Gy was administered in the radiotherapy group and the combination group on day 1, 3 and 5. The mice were sacrificed on day 21, and the tumor size, the activity of cytotoxic T lymphocytes (CTL) and natural killer cells (NK), and the apoptotic activity of the tumor cells were observed. Results: In the combined group the size of tumors was significantly smaller, activity of the NK and CTL was enhanced, and apoptosis was increased, compared to the other groups (P<0.05). The activity of the NK and CTL in the radiotherapy group was reduced. Conclusion: Antitumor immune response can be induced in the radiotherapy group and the LLC/ mIL- 12 vaccine group, but it is much stronger in the combination group. The mechanism of vaccine radiosensitization includes induction of cell apoptosis and enhancement of NK and CTL activity.