Cytochrome P450 (CYP) 3A enzymes are responsible for the metabolism of many drugs. It is useful to know CYP3A activity in individual patients undergoing drug therapy so as to predict the efficacies or adverse events. Lansoprazole is metabolized to Lansoprazole sulfone (LS) by CYP3A, while to 5-hydroxylansoprasole by CYP2C19. The aim of this study was to evaluate whether lansoprazole can be used to assess CYP 3A activity in human liver. Lansoprazole sulfoxidation activity in 14 human liver microsomes was determined as the ratio of lansoprazole/LS, measuring these parameters by high-performance liquid chromatography. Testosterone 6β-hydroxylation (T6β-OH) activity, a known marker for CYP3A activity was also measured together with lansoprazole sulfoxidation activity. Lansoprazole sulfoxidation activity was also analyzed in microsomes preincubat-ed with anti-CYP2C19 antibody. Interindividual variation was observed in lansoprazole sulfoxidation activity and T6β-OH activities of those microsomes, respectively. Lansoprazole sulfoxidation activity was significantly correlated with T6β-OH activity and CYP3A protein level. Lansoprazole sulfoxidation activity in microsomes with anti-CYP2C19 antibody was closely correlated with T6β-OH activity. In contrast, lansoprazole 5-hydroxylation activity was correlated with the CYP2C19 activity. These results suggest that metabolism of lansoprazole to LS by CYP3A occurs independently of metabolism by CYP2C19. LS can be used as a new marker of CYP3A activity.