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Chinese hamster ovary (CHO) cells cultured in vitro were continuously exposed to increasing concentrations of diethylmaleate (DEM). Chronic exposure of these cells (designated CHO/DEM) to 80 microM diethylmaleate resulted in an increase in cystine transport, a decrease in glutathione inhibition of the enzyme gamma-glutamylcysteine synthetase, elevation of intracellular glutathione levels to 4.3 times control, and elevation of glutathione-S-transferase activity by 6.6 times. Yet, CHO/DEM and control CHO cells exhibited the same ability to synthesize protein as measured by two-dimensional electrophoresis, the same cell cycle distribution, and the same population doubling times. CHO/DEM cells are resistant to DEM and diamide cytotoxicity, compared to control CHO cells. CHO/DEM cells were used to address the question of whether chronic elevation of glutathione, above control concentrations, reduced the cytotoxicity produced by exposure to cisplatin, gamma-radiation, or hyperthermia. The resulting dose-response curves obtained with CHO/DEM and control CHO cells indicated that chronic exposure to DEM, which resulted in chronic elevation of glutathione, did not provide protection against any of the three toxic treatments.