Oxidative stress is generated in IL-6–stimulated leukocytes and caused by mitochondria-derived superoxide originated from monocytes. RBC-lysed human leukocytes from healthy donors (n = 6) were activated by IL-6 (10 ng/mL) and incubated with indicated various antioxidants and inhibitors (NAC; a pan antioxidant, L-NMMA; an antioxidant for NOS, Mito-Tempo; an antioxidant for mitochondrial superoxide, 4-ABAH; an antioxidant for MPO, apocynin; an antioxidant for NOX, TXA; an inhibitor of plasminogen activation, AQB-565; anti-inflammatory drug) as described in supplemental Methods and supplemental Figure 8A for 3 hours. Total ROS (A), nitric oxide (NO)(B), and mitochondrial superoxide (C) were measured as described in methods. Each panel is representative of 6 independent experiments. Results in right panel of each row are plotted as the mean fluorescent intensity of each sample on the vertical axis with mean ± SEM. ∗P < .001 vs control, ANOVA, and Bonferroni post hoc test. MPO, myeloperoxidase; NOS, nitric oxide synthase; SEM, standard error of the mean.