Transgenic mouse models have emerged as plausible alternatives to long-term bioassays for carcinogenicity. Three transgenic lines evaluated to date have shown a clear capability to discriminate between carcinogens and noncarcinogens, using long-term bioassay results as the standard. The data also suggest that the transgenic lines will not fully duplicate long-term bioassay results. It is proposed that these models do not respond to chemicals that have induced highly restricted species or strain-specific tumor responses in mice or rats. Rather, the value of the transgenic models is predicated on a preferential response to transspecies carcinogens (i.e., those positive in both rats and mice, often including tumors in the same tissues). Thus, although results in transgenic models may not be completely concordant with long-term bioassays, the data can be used effectively in chemical and drug safety assessments. Further, it is proposed that validation of the models is readily achievable via ongoing studies. Validation of any alternative model is best achieved by sufficient mechanistic understanding of the model to reasonably predict the outcome of bioassays conducted in the models and use all available information on the drug or chemical. This goal can now be met with the transgenic mouse lines.