Adenovirus harboring E. coli. cytosine deaminase gene (AdCD) and adenovirus encoding with lymphotactin gene (AdLtn) were used for gene therapy in vivo. BALB/c mice were inoculated subcutaneously with CT26 colon adeno-carcinoma cells and 3 days later received combined injection of AdCD and/or AdLtn followed by continuous injection with 5-fluorocytosine(5-FC) 300mg/kg. The results demonstrated that mice received combined therapy developed tumors most slowly and survived longest when compared with mice treated with AdCD/5-FC, AdLtn, AdlacZ/5-FC or PBS. To further explain the immunological mechanism of the antitumor effects by the combined therapy, we found that combined treatment with suicide gene and Ltn gene therapy achieved maximal cytotoxic effects of nature killer cells and specific cy-totoxic T lymphocytes. FACS analysis of the tumor mass demonstrated that AdCD/5-FC in combination with AdLtn therapy increased the expression of H-2K~d and B7-1 expression on tumor cells. The CD4~+ and CD8~+ cells infiltrated in the tumor mass after combined therapy were significantly increased when measured by FACS analysis. Our results demonstrated that combined transfer of suicide gene and lymphotactin gene induce nonspecific and specific antitumor immunity of the host and elicit more potent antitumor effect.