It has previously been suggested that adriamycin (doxorubicin, DOX), which is used to treat various types of malignancies, exerts its cytotoxic effects through interactions with DNA and by accelerating membrane lipid peroxidation. Our previous studies have indicated that a mouse glutathione S-transferases isozyme, mGSTA4-4, which shows high activity toward 4-hydroxyalkenals and the hydroperoxides formed during lipid peroxidation, may play an important role in defense mechanism against lipid peroxidation. In order to test the hypothesis that the enhanced protection against lipid peroxidation confers DOX-resistance to cells, we have compared the cytotoxicity of DOX to Chinese hamster ovary cells transfected with mGSTA4-4 with that of controls cells transfected with the vector alone. The results of these studies show that mGSTA4-4-transfected cells have a twofold higher resistance to DOX as compared to the controls. Upon exposure to DOX, the transfected cells showed about 40% less membrane lipid peroxidation as compared to the controls, indicating a positive relationship between DOX-cytotoxicity and lipid peroxidation. These results suggest that mGSTA4-4 provides protection to cells against lipid peroxidation and that DOX cytotoxicity, at least in part, is mediated through oxidative stress resulting in peroxidation of membrane lipids which would explain the results of previous studies showing that DOX can kill cells without being internalized (T.R. Tritton and G. Yee (1982) Science 217, 248-250).