Peroxide accumulation and effects of hydrogen peroxide. (A) Intracellular peroxide accumulation in untreated U-937 cells (Cont), in cells treated for 14 hours with LY294002 alone, and in cells treated for 14 hours with 4 μMAs₂O₃, alone or in combination with LY294002, as determined by flow cytometry after cell loading with H₂DCFDA. The vertical, dotted lines represent the mean fluorescence value in the control, to better discern the displacement caused by the treatments. (B) Frequency of apoptotic cells (mean ± standard deviation of at least 4 determinations) in U-937 cell cultures treated for 24 hours with 20 and 40 μM H₂O₂ alone, and with 1 to 4 μ As₂O₃, camptothecin, and cisplatin, either alone (-) or in combination with H₂O₂. All other conditions were as in Figure 1.