For more than a decade, mutagenicity tests have had a clearly defined role in the identification of potential human mutagens and an ancillary role in the identification of potential human carcinogens. The efficiency of short-term tests in identifying germ cell mutagens has been examined using a combined data set derived from the U.S. Environmental Protection Agency/International Agency for Research on Cancer Genetic Activity Profile (EPA/IARC GAP) and EPA Gene-Tox databases. Our review of these data indicates adequate sensitivity of batteries of in vitro short-term mutagenicity tests in identifying germ cell mutagens. The analysis also supports the inclusion of an in vivo assay as suggested in proposed regulatory testing guidelines. In the context of carcinogenicity testing, the ability of short-term bioassays to detect genotoxic or mutagenic carcinogens is well established. Such tests are not considered to be as sensitive to nongenotoxic or nonmutagenic carcinogens. However, analyses presented in the report using the EPA/IARC GAP database demonstrate that many putative nongenotoxic carcinogens that have been adequately tested in short-term genetic bioassays induce gene or chromosomal mutation or aneuploidy. Further investigation should reveal whether the mutagenicity of these agents plays an important mechanistic role in their carcinogenicity.