This report describes a study by Dr. Ryan Meng and colleagues to determine the role of stereochemistry in the cytotoxicity and mutagenicity of three major butadiene metabolites. This study is a part of HEI's program of research in air toxics; one important aim of that program has been to fill key gaps in risk assessment for some of the most important chemicals. Butadiene is present in motor vehicle exhaust and other emissions and is listed by the U.S. Environmental Protection Agency among the mobile-source air toxics that, because of their emissions and reported toxicity, pose a possible health risk. The study addressed a key research and risk assessment question related to species differences in tumor induction by butadiene, which have been attributed to differences in its metabolism. The investigators evaluated whether different stereoisomers (spatially different forms of the same chemical structure) of the metabolites showed different levels of mutagenicity in a human lymphoblastoid cell line. This study was funded under the Walter A. Rosenblith New Investigator Award.