We are using B6C3F1 lacI transgenic mice to investigate the in vivo mutagenicity and mutational spectra of 1,3-butadiene (BD). Male B6C3F1 lacI transgenic mice were exposed by inhalation to 0, 62.5, 625, or 1250 ppm BD for 4 weeks (6 h/day, 5 days/week). Tissues were collected 2 weeks after the final exposure for mutant frequency analysis. The lacI- mutant frequency in bone marrow from BD-exposed mice was increased 2- to 3.5-fold over air control mice. Analysis of the mutational spectrum of lacI mutants isolated from the bone marrow of lacI mice exposed to 625 ppm BD indicated a shift in the mutational spectrum at A:T base pairs relative to air controls: 2/41 point mutations occurred at A:T base pairs in air controls versus 6/24 in BD-exposed animals; 10/40 mutations occurred at A:T base pairs in lacI mice exposed to 1250 ppm BD. A-->T transversions were found only in BD-exposed animals. Although there was a reduction in the proportion of GC-->AT transitions at 5'-CpG-3' sites recovered from the bone marrow of BD-exposed mice (25%) relative to air controls (50%), there was no difference in the contribution of GC-->AT transitions at 5'-CpG-3' sites to the mutation frequency in BD-exposed mice compared to air controls. Since the bioactivation of BD by B6C3F1 mice produces at least two mutagenic metabolites epoxybutene (EB) and diepoxybutane (DEB), studies on the in vivo mutational spectra begin to provide the framework for the ultimate identification of the BD metabolite(s) responsible for the specific mutations observed.