Previous studies have demonstrated that oxidative stress involving generation of reactive oxygen species (ROS) is responsible for the cytotoxic action of TNFα. Protective effect of small heat shock proteins (small HSP) against diverse oxidative stress conditions has been suggeted. Although overexpression of small hsp was shown to provide an enhanced survival of TNFα-sensitive cells when challenged with TNFα, neither the nature of TNFα-induced cytotoxicity nor the protective mechanism of small HSP has not been completely understood. In this study, we have attempted to determine whether TNFα induces oxidative DNA damage in TNFα-sensitive L929 cells. We chose to measure the level of 8-hydroxy-2′-deoxyguanosine (8 ohdG), which has been increasingly recognized as one of the most sensitive markers of oxidative DNA damage. Our results clearly demonstrated that the level of 8 ohdG increased in L929 cells in a TNFα dose-dependent manner. Subsequently, we asked whether small HSP has a protective effect on TNFα-induced oxidative DNA damage. To accomplish this goal, we have stably transfected L929 cells with mouse small hsp cDNA (hsp25) since these cells are devoid of endogenous small hsps. We found that TNFα-induced 8 ohdG was decreased in cells overexpressing exogenous small hsp. We also found that the cell killing activity of TNFα was decreased in these cells as measured by clonogenic survival. Taken together, results from the current study show that cytotoxic mechanism of TNFα involves oxidative damage of DNA and that overexpression of the small hsp reduces this oxidative damage. We suggest that the reduction of oxidative DNA damage is one of the most important protective mechanisms of small HSP against TNFα.