Phytosterols are plant sterols found in foods such as oils, nuts and vegetables. Phytosterols, in the same way as cholesterol, contain a double bond and are susceptible to oxidation. The objective of the present study was to assess the potential toxic effects of P-sitosterol oxides on U937 cells. The effects of increasing concentrations (0-120 mum) of P-sitosterol oxides on cellular cytotoxicity, apoptosis, antioxidant status and genotoxicity was assessed over 12, 24 and 48 It exposure periods. Following 12 h, the viability of cells treated with 120 mum-beta-sitosterol oxides was reduced to 51.7% relative to control. At 24 and 48 It, both 60 and 120 mum-beta-sitosterol oxides caused a significant decrease in cell viability. For comparison, a decrease in viability of cells treated with a cholesterol oxide, 7beta-hydroxycholesterol (7beta-OH. 30 Vmum), was evident at 24 h. An increase in apoptotic cells, assessed using Hoechst 33342, indicates that the mode of cell death in U937 cells following exposure to 7beta-OH (30 mum) and beta-sitosterol oxides (60 and 120 mum) was by apoptosis. The increase in apoptotic cells after 12 h following treatment with 120 mum-beta-sitosterol oxides was accompanied by a decrease in cellular glutathione. Similarly. 7beta-OH (30 muM) treatment resulted in decreased glutathione at 12 h. Catalase activity was not affected by any of the treatments. beta-Sitosterol oxides had no genotoxic effects on U937 and V79 cells as assessed by the comet and sister chromatid exchange assays respectively. In general. the results indicate that thermally oxidised derivatives of beta-sitosterol demonstrate similar biological effects as 7beta-OH in U937 cells. but at higher concentrations.