The protective role of flavonoids against cholestan-3 beta-5 alpha-6 beta-triol (cholestantriol) induced toxicity was determined. The aims of the present study were: firstly, to investigate the effect of butylated hydroxytoluene (BHT, a synthetic antioxidant), myricetin, quercetin, rutin and cholestantriol on cell viability, endogenous antioxidant enzyme activities and lipid peroxidation in Chinese hamster ovary (CHO) cells and secondly, to determine whether the presence of the flavonoids or BHT could protect against cholestantriol-induced toxicity in vitro. CHO cells were supplemented with either 0-50 mu M cholestantriol, 0-100 mu M of the flavonoids, myricetin, quercetin, rutin or 0-500 mu M BHT. Cytotoxicity was measured by two separate assays, the neutral-red uptake assay and lactate dehydrogenase (LDH, EC 1.1.1.27) release assay. Cholestantriol was the only compound that significantly affected cell viability. CHO cells were exposed to medium containing cholestantriol in the presence or absence of the flavonoids or BHT for a period of 24 h. Lipid peroxidation, as indicated by thiobarbituric acid reactive substances (TBARS) and the activities of the endogenous antioxidant enzymes catalase (CAT, EC 1.11.1.6) and superoxide dismutase (SOD, EC 1.15.1.1) was measured. Cholestantriol significantly reduced antioxidant enzyme activity and increased the extent of lipid peroxidation in the CHO cells (P < 0.05). The addition of the flavonoids or BHT to cholestantriol-supplemented media returned SOD activity back to control levels in the cells. BHT and myricetin restored CAT activity levels in the appropriately supplemented cells. Quercetin and rutin only partially restored the cholestantriol-induced reduction in CAT activity. The flavonoids also reduced the extent of lipid peroxidation in this cellular model. These results suggest that certain flavonoids may play a role in reducing COP-induced toxicities in vitro. (C) 1999 Elsevier Science Inc.