3-Methylindole (3MI) is a naturally occurring pulmonary toxin that requires metabolic activation. Previous studies have shown that 3MI-induced pneumotoxicity resulted from cytochrome P-450-catalyzed dehydrogenation of 3MI to an electrophilic methylene imine (3-methyleneindolenine), which covalently bound to cellular macromolecules. Multiple cytochrome P-450s are capable of metabolizing 3MI to several different metabolites, including oxygenated products. In the present study, the role of human CYP2F1 in the metabolism of 3MI was examined to determine whether it catalyzes dehydrogenation rather than hydroxylation or ring oxidation. Metabolism was examined using microsomal fractions from human lymphoblastoid cells that expressed the recombinant human CYP2F1 P-450 enzyme. Expression of CYP2F1 in the lymphoblastoid cells proved to be an appropriate expression system for this enzyme. Products were analyzed using HPLC and the mercapturate, 3-[(N-acetylcystein-S-yl)methyl]indole, of the reactive intermediate was identified and quantified. Product analysis showed that human CYP2F1 efficiently catalyzed the dehydrogenation of 3MI to the methylene imine without detectable formation of indole-3-carbinol or 3-methyloxindole. High substrate concentrations of 3MI strongly inhibited production of the dehydrogenated product, a result that may indicate the existence of mechanism-based inhibition of CYP2F1 by 3MI. Recombinant CYP2F1 demonstrated remarkable selectivity for the bioactivation of 3MI to the putative dehydrogenated reactive electrophile. Bioactivation of naphthalene to its pneumotoxic epoxide by CYP2F1 was also demonstrated.