H2O2 production was evaluated in liver microsomes prepared from Cyp1a1/1a2(+/+) wild-type and Cyp1a1(−/−) and Cyp1a2(−/−) knockout mice pretreated with 5 μg dioxin (TCDD)/kg body wt or vehicle alone. NADPH-dependent H2O2 production in TCDD-induced microsomes from wild-type mice was about one-third of that in noninduced microsomes. In Cyp1a2(−/−) mice, H2O2 production was the same for induced and noninduced microsomes, with levels significantly higher than those in wild-type mice. Cyp1a1(−/−) microsomes displayed markedly lower levels of H2O2 production in both induced and noninduced microsomes, compared with those in wild-type and Cyp1a2(−/−) microsomes. The CYP1A2 inhibitor furafylline in vitro exacerbated microsomal H2O2 production proportional to the degree of CYP1A2 inhibition, and the CYP2E1 inhibitor diethyldithiocarbamate decreased H2O2 production proportional to the degree of CYP2E1 inhibition. Microsomal H2O2 production was strongly correlated to NADPH-stimulated production of thiobarbituric acid-reactive substances, as well as to decreases in microsomal membrane polarization anisotropy, indicative of peroxidation of unsaturated membrane lipids. Our results suggest that possibly acting as an “electron sink,” CYP1A2 might decrease CYP2E1-and CYP1A1-mediated H2O2 production and oxidative stress. In this regard, CYP1A2 may be considered an antioxidant enzyme.