The synergistic effects of trolox on arsenic-mediated apoptosis are not related to extracellular H₂O₂ production. Cells were treated with As₂O₃ (1 μM) and trolox or ascorbic acid (100 μM) for 48 hours. Catalase (500 U/mL, Cat) was added as indicated to degrade the extracellular H₂O₂ generated. Apoptosis was detected by PI staining and quantitated by flow cytometric measurement of PI-positive cells. Each bar represents an average of 3 independent samples, and standard deviation bars are shown. Asterisks indicate significant differences from As₂O₃ + AA–treated cells (P < .001).