AIM: To evaluate the synergistic antitumor effects of herpes simplex virus thymidine kinase (HSV-TK) together with tumor necrosis factor alpha (TNF-α) or interleukin-2 (IL-2) gene expression on gastric cancer cell line SGC7901.METHODS: Recombinant vectors pL(TT)SN and pL(TI)SN,which express TK-IRES-TNF-α and TK-IRES-IL-2 genes separately, as well as the control plasmids pL(TK)SN and pLXSN were employed to transfect PA317 cells respectively to generate the viruses that can stably express the objective genes through G418 selection. The gastric cancer cells were then transfected by the retroviral serum from the package cells and maintained in culture to determine the cell growth and apoptosis. The cytotoxic effects of HSV-TK together with TNF-α or IL-2 gene expression on the transfected cancer cells were evaluated by the cell viability and bystander effects in the presence of GCV supplemented in the cultural medium.RESULTS: Expression of recombinant proteins including TNF-α and IL-2 by stable transfectants was confirmed by West blotting. The percentage of cell apoptosis in the SGC/0, SGC/TK-TNF-α, SGC/TK-IL-2 and SGC/TK clone was 2.3%, 12.3%, 11.1% and 10.9% respectively at 24 h posttransfection. Cell growth status among all the experimental groups as judged by cell absorbance (,4) at 570nm did not exhibit any significant difference (P＞0.05); although it was noted to be slightly lower in the SGC/-TT group. Cell survival rate in SGC/TI, SGC/TT and SGC/TK group was significantly decreased in a dose-dependent manner of GCV compared with that of the SGC/0 group (P＜0.05-0.01). Among all studied cells, the SGC/TTm was shown most sensitive to GCV rate of SGC/0 cells was not affected by the presence of GCV bystander effect induced by SGC/TI, SGC/TT- and SGC/TK cells was confirmed by the fact that 20% of these stable transfectants could kill 50% of the co-cultured cells, in which the most prominent bystander effect was found in the circumstance of SGC/TTF presence. However, no significant difference of these variables was found among SGC/TI,SGC/TT and SGC/TK cells (P＞0.05).CONCLUSION: The synergistic antitumor effects produced by the co-expression of HSV-TK with TNF-α or IL-2 genes were not present in the transfected SGC7901 cells. The mechanism underlying these phenomena was not known.