The most widely used bioassay in genetic toxicology is the Ames test, which combines a bacterial mutagenicity assay (reversion of Salmonella typhimurium histidine-auxotrophic tester strains) with an exogenous bioactivation system (hepatic postmitochondrial supernatant or "S9"). The enzymatic activities of S9 prepared from the tissues of experimental animals are difficult to control. We show that the requirement for S9 can be obviated by the engineered expression of enzymes of bioactivation within the bacterial cell. With this strategy, reactive metabolites are produced inside the bacterial cell, proximate to the genetic target. Species boundaries can be crossed, and chimeric or mutant enzymes can be studied. We have constructed an Ames tester strain, expressing both aromatic amine N-acetyltransferase and human cytochrome P4501A2, which detects aromatic amine mutagenicity in the absence of S9.