Cytochrome P450 (CYP) enzymes expressed in human lung can metabolize a variety of xenobiotics, drugs, and endogenous compounds. Metabolism of these substrates may lead to their detoxification or activation and may affect the homeostasis of the lung, its susceptibility to disease, response to therapy, and clinical prognosis. We analyzed the expression of CYP2B7, CYP4B1, and NADPH-cytochrome P450 oxidoreductase (OR) mRNAs in normal lung controls, normal lung from lung cancer patients, and lung tumors using the sensitive technique of RNase protection. The mRNAs of CYP2B7, CYP4B1, and OR were detected in all the normal and a majority of neoplastic tissues. The three mRNAs were quantified and found at an average ratio of 0.89, 4.03, and 0.88% relative to actin mRNA in normal lung, respectively. There was no correlation between the levels of expression of the three mRNAs and the histological diagnosis of tumors. The amounts of each of the three mRNAs varied considerably between patients, but analysis of frequency distribution of the levels of CYP2B7 and CYP4B1 mRNAs did not present evidence for genetic polymorphism as a possible source of the observed interindividual variability. Levels of expression of the two P450 mRNAs were reduced (2.3- and 2.4-fold) in the neoplasms compared to normal lung. The level of OR mRNA expression was uniform with no significant differences between normal and neoplastic tissues, and its interindividual variability was the lowest amongst the three mRNAs studied. All mRNAs had increased interindividual variability in neoplastic tissues. Analysis of the patients' smoking histories and the level of CYP2B7, CYP4B1, and OR mRNAs revealed no evidence for their induction by compounds present in cigarette smoke. This study identifies and characterizes lung and lung tumor mRNAs encoding enzymes that may participate in the metabolism of xenobiotics in humans.