The biotransformation of (-)-verbenone was investigated with human liver microsomes by using GC-MS. Regioselective biotransformation was observed when (-)-verbenone was incubated with the liver microsomes. (-)-10-Hydroxyverbenone was formed from (-)-verbenone of kinetic analysis showed that the Km and Vmax values for the hydroxylation of (-)-verbenone by liver microsomes from three human samples, HG-70, HG-56 and HG-23, were 1.1 mM and 4.8 nmol/min/nmol P450, 0.6 mM and 2.1 nmol/min/nmol P450, and 2.8 mM and 4.6 nmol/min/nmol P450, respectively.