Recently-developed transgenic models have provided unprecedented access to rodent somatic and germ line tissues for the study of gene mutation in vivo. While mutations in germ cells are considered an important aspect of any regulatory assessment of the risks posed by chemicals, currently-available conventional tests, which involve the study of thousands of offspring make it impractical to test large numbers of chemicals, for the induction of inherited gene mutations. When effects in germ cells per se, rather than offspring are acceptable targets, transgenic mouse assays may provide a practical alternative. As part of an international collaborative study to begin to determine the reliability, efficacy, and role of such assays, lacZ transgenic mice (Muta Mouse) were treated with single i.p. doses of ethylnitrosourea (ENU), methyl methanesulfonate (MMS), and isopropyl methanesulfonate (iPMS), and mutant frequencies determined using phenyl-beta-D-galactoside (p-gal) positive selection. For studies using germ cells, the selection of sampling times and target cells is crucial. Spermatagonial stem cells and cells in post-spermatagonial stem cell stages are the critical target cell populations of regulatory importance. Cell populations within these categories were studied by sampling germ cells isolated from seminiferous tubules and spermatozoa from the epididymis at 91 days and 25 days after treatment. The data show that ENU and iPMS induced mutations in post-spermatagonial stem cells and spermatagonial stem cells. However, MMS did not induce mutations in either cell type, or at either sampling time, at doses approaching lethality. This result is possibly because MMS induces preferentially large lesions and chromosomal aberrations (as opposed to point mutations), which are not readily detectable with bacteriophage-based shuttle vectors. Since MMS-induced specific locus and dominant lethal mutations are induced only after the mid-spermatid stage, it is also possible that the timing used missed this effect. While the ENU and iPMS data in this study demonstrate the suitability of the lacZ male transgenic mice for the study of gene mutations in post-spermatagonial stem cells and spermatagonial stem cells by sampling cells isolated from seminiferous tubules at selected times after treatment, the MMS results do not answer fully whether transgenic mouse mutation assays can detect mutations resulting from lesions induced after the mid-spermatid stage when most cellular processing is retarded. Nevertheless, it appears clear from presently available information, that the bacteriophage-based lacZ transgenic model is suitable for the detection of gene mutations in spermatogonial stem cells, spermatocytes, and early spermatids.