The effects of interferon and poly I:C on the metabolism and disposition of cyclophosphamide were investigated in mice. Elimination of cyclophosphamide from the blood was decreased in mice treated 24 hr previously with interferon (2.5 x 10(6) U/kg, intraperitoneally) or poly I:C (10 mg/kg, intraperitoneally). The blood half-life of cyclophosphamide in interferon or poly I:C-pretreated mice was prolonged to a first order of disappearance of 29.3 and 41.6 min., respectively, compared to 19.0 min. in control mice. Also, the rate of formation of activated cyclophosphamide was delayed and the peak blood level of activated cyclophosphamide was not as high in poly I:C-pretreated and interferon-pretreated mice as in control mice. The decreased elimination and elevated blood levels of activated cyclophosphamide were reflected by changes in its antitumour activity and toxicity in the mice. In 9,000 x g supernatants prepared from the liver homogenate of mice treated with interferon or poly I:C, the oxidation of cyclophosphamide in vitro has decreased by 29 and 37%, respectively. However, the addition of these agents to normal 9,000 x g supernatant suspensions had no effect on cyclophosphamide oxidation. Modulation of the metabolism and disposition resulted from depressed levels of cytochrome P-450 in the hepatic microsomes of the mice administered interferon or poly I:C.