The elimination of phenytoin from blood was significantly decreased in rats pre-treated 24 h previously with B. pertussis vaccine or the synthetic polynucleotide poly(rl .cntdot. rC). Phenytoin half-life was increased about 4-fold by both agents. In microsomes prepared from rats pre-treated with B. pertussis vaccine or poly(rl .cntdot. rC) the hydroxylation of phenytoin in vitro was decreased by 36 and 53%, respectively. The addition of these agents to normal microsomal suspensions had no effect on phenytoin hydroxylation, demonstrating that the decrease in biotransformation was not due to direct enzyme inhibition. The decrease in phenytoin hydroxylation and elimination rate resulted from depressed levels of cytochrome P-450 in the hepatic endoplasmic reticulum of rats treated with B. pertussis vaccine or poly(rl .cntdot. rC).