Hepatic microsomal cytochromes P-450 CYP1A2, and CYP2E1 contents and catalytic activities have been simultaneously investigated in 42 patients undergoing diagnostic liver biopsy. CYP1A2 contents, measured by Western blotting, were correlated with methoxyresorufin-O-demethylation and ethoxyresorufin-O-deethylation (r = 0.65 and r = 0.66, p < 0.001, respectively). CYP2E1 contents were correlated with 1-butanol oxidation and 6-hydroxylation of chlorzoxazone (r = 0.75 for both, p < 0.001). CYP1A2 catalytic activities varied by 30- to 40-fold, whereas CYP2E1 activities varied by 6- to 20-fold. In our study, these variations were not related to liver diseases or cancer of the digestive tract nor to alcohol drinking or smoking habits, because patients were alcohol- and tobacco-free for 1 month before the study. Other environmental factors, diet habits, and/or genetic factors could explain the large interindividual variations observed.