Oxysterols have been shown in a number of cell lines to induce apoptosis by a mechanism as yet unclear. The induction of apoptosis by certain agents has been associated with the generation of oxidative stress and the depletion of the endogenous antioxidant, glutathione, which may result in cytochrome c release and caspase activation. The aim of the present study was to determine whether 7 beta-hydroxycholesterol (7 beta-OH) alters glutathione levels or the activities of catalase, superoxide dismutase (SOD) or caspase-3 in association with cell death in either the U937 or the HepG2 cell lines. 7 beta-OH, which induced significant apoptosis at 12 h in the U937 cell line, was shown to cause a significant decrease in glutathione levels and an increase in the activity of SOD at this time point. An increase in caspase-3 activity was also observed in the U937 cell line following a 24-h incubation with 7 beta-OH. Glutathione concentration, SOD activity and caspase-3 activity were unchanged in the HepG2 cell line, which underwent necrosis following incubation with 7 beta-OH. The activity of the enzyme catalase remained unchanged in both cell lines. These results provide evidence that the generation of an oxidative stress may be a significant event occurring during 7 beta-OH-induced apoptosis.