To assess the contribution of cytochrome P450 to the microsomal ethanol oxidizing system in humans, we examined ethanol oxidation activity using multiple forms of human hepatic P450s purified from yeast cells with cDNA-expressed P450. The ethanol oxidation activity by ten forms of purified human P450s was investigated with a reconstituted system. P450 2E1 and 1A2 formed acetaldehyde at high rates. In immunoinhibition studies with human hepatic microsomes, anti-P450 2E1 antibody inhibited the ethanol oxidation activity by 54% and anti-P450 1A2 antibody inhibited ethanol oxidation activity by 21%. Additionally, when both of P450 2E1 and 1A2 antibodies were added to the reaction system, the activity was inhibited by 68%. 7,8-Benzoflavone, an inhibitor of P450 1A forms, also inhibited this activity by 38%. The correlation of ethanol oxidation activity with the levels of immunoreactive P450 1A2 in individual human microsomes was highly significant (r 2 =0.76, p<0.001). The Vmax/Km value of P450 1A2 for ethanol oxidation was 2.2, that was higher than that of P450 2E1(1.5). These results indicate that P450 2E1 is a major contributor to microsomal ethanol oxidizing system in human, however, P450 1A2 is also considered to play an important role in the microsomal ethanol oxidizing system.