Examination of a profile of data already existing on 1,3-butadiene shows adequate knowledge in many areas of toxicology that are conventionally required in hazard identification. However, while much progress has been made in areas of metabolism and pharmacokinetics, further studies would be worthwhile to improve mechanistic understanding such as the examination of alternate metabolic pathways, the generation of interspecies scaling factors, and an assessment of the relevance of various tumor sites. In this respect, data pertaining to repeated and pulse exposures of rodents and primates would be helpful. Another important aspect is the need to understand any human health implications of the observed 1,3-butadiene-induction of the murine leukemia virus. In this respect, studies have been pursued that include the comparison of leukemogenesis in congenetic strains, the leukemogenicity of viral isolates in rodent carcinogenicity and human cell culture studies, and the mechanisms of activation of ecotropic proviral sequences. Molecular epidemiological and toxicological research is ongoing in rodents and primates to evaluate hemoglobin adduct formation as an index of 1,3-butadiene exposure. Challenges of specificity, sensitivity, and simplification of current procedures need to be overcome. Recent mutagenicity and metabolism data suggest that structurally-related isoprene may have carcinogenic potential. The use of interstrain comparative studies and data in a second species is discussed, as well as proposed metabolism studies.