[Species differences in the in vitro metabolism of 2,4,5,2',3',4'-hexachlorobiphenyl]. 
In vitro Metabolism of 2,4,5,2',3',4'-hexachlorobiphenyl (CB138) was studied using liver microsomes from rats, hamsters and guinea pigs.  Guinea pig liver microsomes formed four metabolites named as M-1, M-2, M-3 and M-4 and these metabolites were all increased to about 4-5 fold of untreated microsomes by pretreatment of phenobarbital.  Liver microsomes of rats and hamsters showed much less activity to metabolize CB138 than those of guinea pigs.  Only phenobarbital-treated microsomes produced very small amounts of M-3 in rats and M-1, M-2 and M-3 in hamsters, but untreated and MC-treated microsomes did not.  When mass spectra of the methylated derivatives of M-1, M-2, M-3 and M-4 were measured in GC/MS, the former two possess the molecular ion of 354 and the latter two had the molecular ion of 388.  In addition, the mass fragmentation pattern indicated that M-1, M-2, M-3 and M-4 are 2-OH-4,5,2',3',4'-pentachlorobiphenyl, 5-OH-2,4,2',3',4'-pentachlorobiphenyl, 3-OH-CB138 and 2-OH-3,4,5,2',3',4'-hexachlorobiphenyl, respectively.  Of four metabolites, the chemical structures of M-3 and M-4 were supported by the synthesized authentic compounds.  From these results, it is suggested that the metabolism of CB138 in guinea pig liver proceeds mainly via 2,3-epoxide as an intermediate and a PB-inducible P450, CYP2B18, is the most important isozyme in CB138 metabolism.