To identify environmental carcinogens there is a need for inexpensive and reliable short-term tests, but certain human or animal carcinogens are persistently undetectable as mutagents with the Ames assay or with other short-term tests currently in use. Thus there is a need for short-term tests which detect carcinogens missed by the Ames assay. Because of the association of carcinogenesis with genome rearrangement, a system screening for intrachromosomal recombination resulting in genome rearrangement has been constructed in Saccharomyces cerevisiae. Evaluation of this system shows inducibility by a variety of carcinogens not detectable by the Ames assay or various other short-term tests. In the light of these results it is tempting to speculate that 'nongenotoxic carcinogens' are in fact genotoxic but, in the past, the tools to measure the genetic alterations they induce have been inappropriate.