Treatment of aerobic Escherichia coli K 12 with nonlethal doses of near-ultraviolet (NUV) and H2O2 resulted in the same physiological response. In response to stress caused by these agents, an inhibition of growth, a decrease in intracellular K+ pool, and an increase in intracellular levels of protein and nonprotein thiols were observed. A possible role for glutathione in the process of adaptation of E. coli culture to growth under the conditions of oxidative stress is discussed.