TNF induces intracellular oxidation in U937. DCF-DA–loaded cell cultures were left untreated or stimulated with 20 ng/mL TNF in the absense or presence of pretreatment with antioxidant (2% [vol/vol] DMSO, 7.5 mmol/L NAC, or 4 mmol/L vitamin C; determined to be effective concentrations in preliminary experiments). Mean fluorescence intensity, each point representing 10⁴ cells, is plotted as a function of time. The error bars represent SD from 3 independent experiments. Statistical analysis of the TNF-only stimulated cells (at 12, 15, and 18 minutes) versus unstimulated or versus TNF plus antioxidant, yields P < .0001.