Cytochrome P450 2E1 (CYP2E1) is believed to have a significant role in the bioactivation of 1,3-butadiene (BD) to DNA reactive epoxide metabolites that induce somatic and germ cell genotoxicity in mice. To assess the potential role of in situ bioactivation of BD by mouse testes for inducing germ cell genotoxicity, the presence of CYP2E1 in testes has been demonstrated by reverse transcriptase-polymerase chain reaction (RT-PCR), immunoprecipitation-Western blotting methods (IP-Western) and immunohistochemistry of tissue sections. Detection of CYP2E1 in the testes was limited to interstitial cells. In liver a known site of BD bioactivation and a positive control tissue used for these studies, a discrete, zonal staining pattern of liver CYP2E1 expression detected by immunohistochemical staining was shown. These results suggest that in situ bioactivation of BD in testes by CYP2E1 may contribute to BD-induced germ cell genotoxicity.