Effects of schisandrin B enantiomer ((+)Sch B and (-)Sch B) treatment on the reduced cellular glutathione (GSH) level and susceptibility to menadione-induced toxicity were investigated and compared in AML12 hepatocytes. (+)Sch B or (-)Sch B treatment at 6.25 micromol/l produced a time-dependent change in cellular GSH level, with the maximal stimulation occurring 16 h after dosing. (+)Sch B/(-)Sch B pretreatment for 16 h dose-dependently protected against menadione toxicity, with the maximum degree of protection observable at 6.25 micromol/l and the extent of protection afforded by (-)Sch B being larger than that of (+)Sch B. The cytoprotection was associated with a parallel enhancement in cellular GSH level in both non-menadione (control) and menadione-intoxicated cells. While the GSH depletion produced by buthionine sulfoximine/phorone treatment largely abrogated the cytoprotective action of (+)Sch B/(-)Sch B, it almost completely abolished the GSH-enhancing effect of (+)Sch B and (-)Sch B in both control and menadione-treated cells. Both (+)Sch B and (-)Sch B treatments increased the GSH reductase activity in control and menadione-treated cells, with the stimulatory action of (-)Sch B being more potent than that of (+)Sch B in the control condition. (+)Sch B and (-)Sch B also enhanced the gamma-glutamate cysteine ligase activity in menadione-intoxicated cells. The results indicate that (-)Sch B is more effective than (+)Sch B in enhancing cellular GSH and protecting against oxidant injury in hepatocytes.