Since the metabolism of polychlorinated biphenyls (PCBs) is the critical factor that determines whether or not they accumulate in adipose tissue, we have studied the metabolism of two hexachlorobiphenyls (HCBs), 2,2'3,3',6,6'-hexachlorobiphenyl (236-HCB) and 2,2'4,4',5,5'-hexachlorobiphenyl (245-HCB), by human hepatic microsomes. Human microsomes were isolated from patients undergoing liver resection and were found to have cytochrome P-450 levels (0.28 nmoles/mg microsomal protein) and cytochrome P-450-dependent enzymatic activities similar to those reported by other workers. 245-HCB was not metabolized by human microsomes under various conditions, while 236-HCB was metabolized with an apparent Km of 8.8 microM and a Vmax of 5.1 pmoles/mg microsomal protein/min. Two major metabolites were formed and identified by gas chromatography-mass spectrometry as 2,2',3,3',6,6'-hexachloro-4-biphenylol and 2,2',3,3'6,6'-hexachloro-5-biphenylol. [14C]236-HCB equivalents were found to covalently bind to microsomal protein. Addition of 1 or 5 mM reduced glutathione decreased the degree of covalent binding. These data suggest that HCBs are metabolized through an arene oxide. The fact that 245-HCB was not metabolized explains why it is the predominant PCB found in human adipose tissue.