The purpose of this study was to investigate the expression and distribution of pulmonary CYP2E1 in mice. The CYP2E1 protein and mRNA were identified by immunoblotting and northern blotting, respectively, while the distribution of the CYP2E1 protein and mRNA was examined by immunohistochemistry and in situ hybridization, respectively. Protein immunoblotting revealed a single band of approximately M(r) 51,000 in lung microsomes of CD-1 male mice. Northern blotting with a 32P-labeled RNA probe for CYP2E1 detected a single species of approximately 2 kb that was similar in size to that of liver CYP2E1. Immunohistochemical studies with the avidin-biotin complex procedure showed that CYP2E1 was localized prominently in the nonciliated Clara cells but was not detected in the ciliated cells of the bronchiolar epithelium. In the lung parenchyma, immunoreactivity for CYP2E1 was evident at minimal levels in alveolar type II cells. In situ hybridization experiments with a 33P-labeled RNA probe showed that the CYP2E1 mRNA was also predominantly localized in the bronchiolar epithelium and was most prominent in the Clara cells. As was found for the CYP2E1 protein, the CYP2E1 mRNA was minimal in cells of the lung parenchyma. These results demonstrated that the CYP2E1 enzyme is preferentially expressed in Clara cells of murine lung. The concentration of CYP2E1 mainly in this cell population may be an important determinant underlying its susceptibility to cytotoxicities induced by xenobiotics bioactivated by this P450 isozyme.