OF WORK: Until recently, in vivo mutagenesis studies in humans or in mice used the X-linked, Hprt , gene. Unfortunately, the use of Hprt as a marker for in vivo mutagenesis studies is essentially limited to the T-cell lymphocyte population. Therefore, several transgenic mice have been generated that allow for the efficient recovery of a transgene, thereby providing a relatively simple genetic system for the assessment of mutations in virtually any tissue. We use the Big Blue? transgenic mouse which carries 30-40 genomic copies of the lambda-LIZ phage. The lacI gene, which is carried on lambda-LIZ, has been used most often as a target gene for molecular studies. More recently, however, the lambda-cII gene has also been defined as a useful mutational target gene. Both genes are present in Big Blue? mice allowing for comparative in vivo mutagenesis studies. We have characterized 978 spontaneous mutations recovered from various tissues of 6-8 week old male Big Blue? mice. The spectra recovered from two strains of mice, either C57Bl/6 or B6C3F1, were compared and found to be very similar. Mostly, these data were generated in the B6C3F1 mouse strain which is commonly used in the National Toxicology Program (NTP) 2-year carcinogenesis bioassay. Mutant frequencies ranged from 2.5 x 10e-5 to 7.1 x 10e-5 for liver, spleen, bladder, stomach, kidney, bone marrow, lung and skin. GC to AT transitions at CpG sites predominate the specturm in all tissues. In order to evaluate the cII as a useful alternative to lacI as a mutational target locus, we initiated a study to determine the spontaneous and ENU-induced mutant frequencies (MF?s) and mutational spectra at cII using Big Blue? Rat2 embryonic fibroblasts. Overall, the spontaneous cII mutational spectrum differs slightly from spontaneous spectra reported at the Big Blue? lacI locus; but the mutational spectra and base substitution mutant frequencies following treatment with ENU were comparable at both loci. These data support the continued use of cII as a selectable marker in mutagenesis studies involving cells or tissues that carry a l transgene. Finally, we have characterized i) the ploidy, ii) the mitotic index, iii) the spontaneous frequencies of chromosome and chromatid aberrations and iv) the rate of micronucleus (MN) formation in both the Rat2 and Big Blue? mouse embryonic fibroblast cell lines. Both cell lines were shown not to be useful for routine cytogenetic toxicology studies. As a result, new transgenic Big Blue? primary rat and mouse fibroblast cell lines have been established for use in future in vitro/in vivo comparative studies.