We have investigated the effect of a range of 2-nitroimidazoles on CCNU metabolism, using an in vitro mouse liver microsomal preparation. CCNU is hydroxylated to at least 5 monohydroxylated metabolites. For the major metabolite, cis-4-hydroxy CCNU, values of Km and Vmax were 0.026 mM and 1.92 nmol/min/mg protein, respectively. MISO and other 2-nitroimidazoles inhibited the hydroxylation of CCNU in a dose-dependent manner and their potencies as inhibitors were governed by their lipophilicities. In order of increasing potency I50 values were 15.5, 6.4, 5.8, 1.4, 0.4, and 0.37 mM for SR 2508, Ro 03-8799, MISO, Ro 07-1902, Ro 07-1127, and BENZO, respectively. Chemosensitization potency correlated well with the extent of inhibition at achieved plasma concentrations in mice, suggesting a causal relationship between enzyme inhibition and chemosensitization. All the nitroimidazoles exhibited type II optical difference spectra with phenobarbitone-induced mouse liver microsomes. However, with increasing lipophilicity of the nitroimidazole both the wavelength at maximum absorbance (lambda max) and the isosbestic point of the type II spectrum were shifted to longer wavelengths, suggesting that a type I binding component may become more significant. Our previous work has shown that changes in CCNU pharmacokinetics contribute to chemosensitization by nitroimidazoles in mice, and that altered pharmacokinetics also occur in man. The present results provide strong evidence that the mechanism involves binding to liver microsomal cytochrome P-450, leading to inhibition of CCNU metabolism.