The effect of intratracheal administration of fly ash, benzene extracted fly ash residue and benzene extract of fly ash has been studied on the activity of pulmonary mixed function oxidase. Fly ash, its benzene extract and benzene extracted residue significantly increased the levels of cytochrome P-450, cytochrome b5 and the activities of NADPH-cytochrome c reductase, NADH cytochrome b5 reductase, aminopyrine N-demethylase and glutathione S-transferase in a dose dependent manner. Phenobarbital or 3-methylcholanthrene treatment along with administration of fly ash, its benzene extracted residue and benzene extract of fly ash showed a synergistic effect on the activity of mixed function oxidase. The observed effects were due to chemical causes, i.e. organic and inorganic fractions of fly ash and not, due to its particulate nature. This was shown by the administration of glass beads which caused no alteration in the activity of pulmonary mixed function oxidase.