Objectives : Achyranthes bidentata radix (Usul) has been used as anti-arthritic, antiallergic, antidiuretic, and so on. Recently extracts of Achyranthes bidentata radix have shown anti-inflammatory and cancer preventive effects in vitro and in vivo. Methods : We therefore evaluated the inhibitory potential of ethanol extracts of Achyranthes bidentata radix on cytochrome P450 (CYP) isoforms-catalyzed reactions, which relate to causes of cancer and inflammation, including CYP1A2, CYP2C9, CYP2C19, CYP2E1, CYP2D6, CYP2C8, and CYP3A4, using human liver microsomal preparations. Results : The extracts showed weak or negligible inhibitory effects on CYP2C9-catalyzed (S)-warfarin 7-hydroxylation, CYP2C19-catalyzed S-mephenytoin 4-hydroxylation, and CYP2D6-catalyzed dextromethorphan O-demethylation with each IC50 over 1750 g/ml, respectively. However, it showed relatively significant inhibitory effect on CYP1A2-catalyzed phenacetin O-deethylation and CYP2E1-catalyzed chlorzoxazone 6-hydroxylation with IC50s of 970.5 g/ml and 821.4 g/ml, respectively. Conclusions : These results suggest that extracts of Achyranthes bidentata radix have inhibitory effects on CYP-catalyzed reactions, especiallyCYP1A2 and CYP2E1, in human liver microsomes. These effects appear to relate to anti-inflammatory and cancer prevention following decrease of reactive oxygen species formed by CYP, especially CYP1A2 and CYP2E1, by Achyranthes bidentata radix. However, further evaluation is necessary to demonstrate and to confirm its effects in human.