The effects of human recombinant interferons alpha and gamma on monooxygenase activities in cultured human hepatocytes have been investigated. Dose-response and time course studies showed that interferons reduced 7-ethoxyresorufin O-deethylase activity of human hepatocytes after a 12-hr incubation with 300 U/ml interferons alpha and gamma (52% and 38% decrease, respectively). A reduction in 7-ethoxyresorufin O-deethylase activity was also observed in HepG2 cells, although in these cells maximal inhibition was observed after 24 hr of treatment with 1000 U/ml (a 41% and 28% decrease with interferon alpha and gamma, respectively). A decrease in activity was also observed in 7-pentoxyresorufin O-depentylase, 7-ethoxycoumarin O-deethylase and testosterone 2alpha- and 6beta-hydroxylase. It is noteworthy that the marked increase in 7-ethoxyresorufin O-deethylase activity detected in human hepatocytes after incubation with 2 mum-3-methylcholanthrene (10-fold over non-treated cells) was reduced by 40% in the presence of interferons (300 U/ml), thus indicating that the inducibility of monooxygenases could be altered by interferon treatment. The inhibitory effect of interferons on 7-ethoxyresorufin O-deethylase was transient and HepG2 cells recovered their normal activity 24 hr after interferon removal from culture medium. This study provides the first direct evidence that interferons down-regulate the level of monooxygenases in human hepatic cells and prevent, in part, their induction by xenobiotics.