AIM: To test the efficacy of gene therapy in rat liver tumor.METHODS: A retroviral vector GCIL12EIL2PN encoding human IL-2 (hIL-2) and mouse IL-12 (mIL-12) fused gene and its packaging cell were constructed. The packaging cell lines contained of IL-2 and/or IL-12 genes were injected intrasplenically to transfect splenocyte at different time.The therapeutic effect, immune function and toxic effect were evaluated.RESULTS: The average survival times of the 4 groups using IL genes at days 1, 3, 5 and 7 after tumor implantation were 53.3±3.7, 49.3±4.2, 31.0±2.1 and 24.3±1.4 d respectively in IL-2/IL-12 fused gene group, 25.0±2.5, 23.5±2.0,18.3±2.4 and 12.0±1.8 d respectively in IL-2 gene treatment group, and 39.0±4.8, 32.0±3.9, 23.0±2.5 and 19.4±2.1 d respectively in IL-12 gene treatment group (P＜0.01, n=10).In the IL-12/IL-2 fused gene treatment group, 30% of rats treated at days 1 and 3 survived more than 60 d and serum mIL-12 and hIL-2 levels were still high at day 3 after treatment. Compared with IL alone, NK cell activity was strongly stimulated by IL-2/IL-12 gene. Microscopy showed that livers were infiltrated by a number of lymphocytes.CONCLUSION: IL-2 and/or IL-12 genes injected directly into spleen increase serum IL-2 and IL-12 levels and enhance the NK cell activity, which may inhibit the liver tumor growth. The therapy of fused gene IL-2/IL-12 is of low toxicity and relatively high NK cell activity. Our data suggest that IL-2/IL-12 fused gene may be a safe and efficient gene therapy for liver tumor. The gene therapy should be administrated as early as possible.