The in vitro metabolism of (−)-camphor was examined in human liver microsomes and recombinant enzymes. Biotransformation of (−)-camphor was investigated by gas chromatography-mass spectrometry (GC-MS). (−)-Camphor was oxidized to 5-exo-hydroxyfenchone by human liver microsomal cytochrome (P450) enzymes. The formation of metabolites of (−)-camphor was determined by the relative abundance of mass fragments and retention time on gas chromatography (GC). CYP2A6 was the major enzyme involved in the hydroxylation of (−)-camphor by human liver microsomes, based on the following lines of evidence. First, of eleven recombinant human P450 enzymes tested, CYP2A6 catalyzed the oxidation of (−)-camphor. Second, oxidation of (−)-camphor was inhibited by (+)-menthofuran and anti-CYP2A6 antibody. Finally, there was a good correlation between CYP2A6 contents and (−)-camphor hydroxylation activities in liver microsomes of 9 human samples.