Objective: To investigate the therapeutic effects of the murine IL-12 (mIL-12) retrovirus packaging cell line on hepatoma injected locally. Methods: The retrovirus vector encoding mIL-12 gene was constructed and transfected into packaging cell line PA317. The cells were then used to treat the rats with experimental orthotopic hepatoma at different time. The therapeutic effects, immune functions of the hosts, pathological and toxicological responses were documented. Results: the results showed that the mIL-12 retrovirus packaging cell line could significantly inhibit the growth of the hepatoma cells injected locally to the hepatoma. The early treatment made the rats survive long, while the medium or late stage treatment could prolong the life time of the rats compared with the bland control group or bland vector control group, though the rats did not survive. The number of NK cells and T cells increased significantly in the treatment group. The effects of the early treatment were superior to those of the medium and late stage treatment. Moreover, the transfection of IL-12 gene locally in the hepatoma tissue could make the hepatoma disappear from other liver lobe. This phenomenon demonstrated that IL-12 could activate the immune cells of the host to kill the untransfected tumor cells. This is very important for IL-12 to be used in gene therapy clinically. Meanwhile, the hepatoma would not recur in the rats that had survived more than 2 months from the early treatment after being re-challenged with tumor cells. Conclusion: the results showed that IL-12 gene injected locally in the hepatoma tissue could enhance the anti-tumor immunity of the host.