Styrene is known to cause both hepatotoxicity and pneumotoxicity in mice. Strain differ ences have been reported by other investigators suggesting that Swiss mice are less suscep tible than non-Swiss mice to styrene-induced liver damage. In this study, A/I and C57BL/6 mice were found to be similar to non-Swiss albino (NSA) mice in susceptibility whereas CD-1 (Swiss) mice were more resistant to hepatotoxicity as assessed by serum sorbitol de hydrogenase levels and pneumotoxicity as determined by gamma-glutamyltranspeptidase and lactate dehydrogenase measurements in bronchoalveolar lavage fluid. Styrene was hepatotoxic in CD-I mice treated with pyridine to induce CYP2E1. CYP2EI apoprotein levels and p-nitrophenol hydroxylase activities in control and pyridine-induced mice were similar in the two strains. Hepatic and pulmonary microsomal preparations from both strains metabolized styrene to styrene oxide at similar rates. CD-1 mice were as susceptible as the NSA mice to the effects of styrene oxide. The data suggest that there are no differ ences in the bioactivation of styrene to styrene oxide or innate susceptibility to the active metabolite that would account for the differences between the CD-1 and NSA mice.