The transgenic mouse strain 40.6 (Muta Mouse) was developed for the detection of gene mutations induced in vivo. Strain 40.6 was constructed by microinjecting the shuttle vector lambda gt10 lacZ into the male pronucleus of a single cell embryo resulting from a CD2 (i.e. BALB/c x DBA/2)F1 x CD2F1 cross. Approximately 40 concatenated copies of the shuttle vector were integrated per haploid genome. The resulting mice were bred to disomy for the insert for use in mutagenicity studies. Ultimately, it is hoped that transgenic rodent model systems such as this one will play an important regulatory role in hazard identification. Despite the increasing use of this strain in toxicological studies, relatively little is known about the site of integration of the target gene into the mouse genome. In this study, fluorescence in situ hybridization and DAPI chromosome banding were combined to determine the location of the transgenic element in the mouse genome. The results indicate that the lambda sequences containing the lacZ gene are located in the B region of mouse chromosome 3. No other major chromosomal rearrangements were evident in the genome of this mouse strain.