Human recombinant interleukin-2 (IL-2) administration is being tested in patients with advanced cancer. Its effects on the expression of cytochromes P-450 were determined in rats. IL-2 administration (1-25 x 10(6) U/kg i.v. twice daily for 1 to 4 days) resulted in a time- and dose-dependent decrease in cytochrome P-450 measured by the absorbance of its Fe(++)-CO complex. After 25 x 10(6) U/kg twice daily for 4 days, cytochrome P-450 decreased 44%; immunoreactive cytochrome P-450 1A1 decreased nonsignificantly (22%); but cytochrome P-450 1A2 decreased 68%; 2B1/2, 50%; 2C11, 75%; 2D1, 36%; and 3A, 70%. Aminopyrine N-demethylase activity decreased 53%, ethoxycoumarin O-deethylase 64%, benzo(a)pyrene hydroxylase 71%, ethoxyresorufin O-deethylase 42%, pentoxyresorufin O-dealkylase 81% and erythromycin N-demethylase 56%. In rats treated with 3-methylcholanthrene for 4 days, IL-2 coadministration (25 x 10(6) U/kg i.v. twice daily for 4 days) did not decrease significantly immunoreactive cytochrome P-450 1A1 and 1A2, whereas cytochromes P-450 2B1/2, 2C11 and 3A decreased 39, 54 and 67%, respectively. In rats treated with phenobarbital for 4 days, IL-2 coadministration decreased immunoreactive cytochromes P-450 2B1/2 29%, whereas cytochromes P-450 1A2, 2C11 and 3A decreased 38, 63 and 67%, respectively. We conclude that administration of high doses of IL-2 decreases the expression of several cytochromes P-450 in rats. Microsomal enzyme inducers appear to limit the effects of IL-2 on the induced forms of cytochromes P-450. Because much lower doses are used in humans, their potential effects on drug metabolism cannot be assessed from present results.