Induction of the monooxygenase system of xenobiotic metabolism by means of hexachlorobenzene (HCB) was studied in rat liver microsomes as compared with induction using phenobarbital (PB) and 3-methylcholanthrene (3-MC). HCB was shown to increase the content of cytochrome P-450 as well as aminopyrine- and benzphetamine-N-demethylase activities in liver microsomes. At the same time, HCB increased the benzpyrene hydroxylase activity as it was the case in induction with 3-MC. Electrophoresis in SDS-polyacrylamide gel showed that HCB, as phenobarbital does, induced appearance of the protein with a molecular mass 52,000 (cytochrome P-450) but not of the protein of molecular mass 56,000, which is the main isozyme of cytochrome P-450 in 3-MC microsomes (P-448). Except of cytochrome P-450, cytochrome, immunologically similar to cytochrome-P-448 from 3-MC microsomes, constituted 10% of the total content of cytochrome in HCB microsomes as shown by rocket immunoelectrophoresis using monospecific antibodies towards individual cytochromes P-448 and P-450 (anti-P-448 and anti-P-450). Anti-P-450 and especially anti-P-448 inhibited benzpyrene hydroxylase in HCB microsomes. HCB appears to be an inductor of a 'mixed' type.