A microtiter-plate assay has been developed for total intracellular glutathione that facilitates multiple-sample analysis and reduces the amount of time and chemicals required. Sonication time, pH, and storage conditions were identified as key parameters that affect the accuracy of the assay. Using this assay, it was found that CHO cells increase their glutathione level under higher oxygen tension. This adaptive response suggests that a rise in glutathione may be used as an indicator of oxidative stress. Based on this criterion, it was found that hypoxic and anoxic cells are sensitized to reoxygenation. This sensitization could not be attributed to a drop in glutathione during low oxygen exposure because the glutathione content reached a basal level at a PO2 of about 40 torr.