The radiation responses at various oxygen tensions were evaluated in V79 Chinese hamster cells under conditions where their nonprotein thiols, primarily glutathione (GSH), were elevated by 2-oxothiazolidine-4-carboxylate (OTZ). OTZ, when cleaved by intracellular oxoprolinase, provides the cell with cysteine which stimulates GSH synthesis. A 2-hr pretreatment with 10 mM OTZ elevated GSH to 200% of controls. This elevation in GSH offered no protection to aerated cells; however, for O2 tensions less than or equal to 40,000 ppm modest protection was observed as evidenced by an increase in oxygen enhancement ratio. GSH elevation afforded maximal protection between 1000 and 10,000 ppm O2; however, the extent of protection was relatively small (protection factor = 1.3).