Effect of 6-MFA (sixth mycelial fraction of acetone), an interferon inducer obtained from fungus A. ochraceus on hepatic mixed function oxidase system (MFO) of rat has been investigated. Treatment with 6-MFA, 100 mg/kg/day, ip for 1-5 days to adult rats inhibited significantly the different indices of MFO system, viz. hepatic cytochrome P-450, cytochrome b5 content, cytochrome c reductase, aminopyrine-N-demethylase and acetanilide hydroxylase activities. Similar treatment for 3 days in young growing rats significantly inhibited MFO system's components except acetanilide hydroxylase activity which showed marked elevation. These effects seem to be specific as in vitro experiments suggested that 6-MFA does not compete with subsdtrates nor it acts as a sponge reacting with the end product to give false inhibitory effect. It is concluded from the present study that 6-MFA like other interferon inducers depresses MFO system in rats. Its possible clinical implications are discussed.