Background: Since tumor necrosis factor was discovered in 1975, TNF has been well known about its cytotoxic effect on tumor cells in vivo and in vitro. According to the recent improvement of molecular biological techniques, it is possible that exogenous TNF gene is transferred to tumor cells and is expressed in theirs. By virtue of TNF gene transfer, we have expected that TNF expressed in TNF-gene-transferred tumor cells would kill tumor cells in vivo without systemic side effect. The expected mechanisms in which antitumor effects of TNF expressed in TNF-gene-transferred tumor cells are working would be as followings. In the first mechanism, TNF expressed in TNF-gene-transferred tumor cells would kill tumor cells around (like homicide). In the second mechanism, TNF expressed in TNF-gene transferred tumor cells would kill themselves (like suicide). In the third mechanism, TNF expressed in TNF-gene-transferred tumor cells would recruit immune effector cells and kill tumor cells indirectly. In the last mechanism, TNF expressed in TNF-gene-transferred tumor cells would augment cytokine such as interferon-γ to kill tumor cells. Among these four mechanisms of antitumor effect, only the second mechanism has not been established yet. Therefore, to elucidate the second mechanism. We performed this study. Method: We transferred TNF-α gene to NCI-H2058, a human mesothelioma cell line and WEHI164, a murine fibrosarcoma cell line by using retroviral vector(pLT12SNTNF). And, We determined by using MTT assay whether TNF expressed in TNF-gene-transferred tumor cell lines would kill themselves like suicide or not. Then, if TNF-gene-transferred tumor cell lines would not suicide themselves, I would know more about the TNF sensitivity of TNF-gene- transferred tumor cell lines to exogenous TNF also by MTT assay. Result: NCI- H2058 and WEHI164 which were sensitive to TNF, became far less sensitive to endogenous and exogenous TNF after being transferred TNF-α gene to. Conclusion: TNF-gene-transfer to NCI-H2058 and WEHI164 gave them resistance to TNF.