Chronic exposure to 1,3-butadiene (BD) results in early occurrence and high incidence of lethal lymphomas in male B6C3F1 mice. Male B6C3F1 lacI transgenic mice (BigBlue) were exposed by inhalation to 0, 62.5, 625 or 1250 p.p.m. BD for 4 weeks (6 h/day, 5 days/week). The lacI- mutant frequency and mutational spectrum were evaluated in DNA isolated from bone marrow cells. Two weeks after exposure the lacI- mutant frequency in bone marrow from BD-exposed mice had increased 2- to 3.5-fold over air control mice. DNA sequence analysis of 56 and 54 lacI- mutants from the air control and butadiene-exposed groups, respectively, demonstrated that there was a shift in the spectrum of base substitution mutations at A:T sites in BD-exposed mice (6/26) compared to the air control mice (2/45). A:T-->T:A transversions were found only in BD-exposed animals. Sequence data also indicate that clonal expansion, a natural process in hematopoiesis, can contribute to the lacI- mutant frequency from bone marrow cells such that mutant frequency and mutation frequency are not equivalent. This study shows that BD is mutagenic in B6C3F1 transgenic mouse bone marrow, causing a shift in the mutation spectrum at A:T base pairs in BD-exposed mice compared to air control mice. Sequencing DNA from lacI- mutants from transgenic animals, to determine mutant/mutation frequency and mutational spectra after subchronic bioassay-like exposure conditions, will aid in linking exposure, dose and biological response for genetic risk assessment of BD.