The aroC321 allele in Salmonella typhimurium permits a positive selection for genetic duplications. Bacteria that contain a large genetic duplication are detected as tryptophan prototrophs in aroC321 strains and occur at a spontaneous frequency greater than 1/10(4) cells plated on the selection medium. Duplications originate by a recombinational mechanism, and the induction of duplications by chemicals or radiation may therefore be considered to be a recombinagenic effect. We have found that strychnine is a potent recombinagen in this system; it causes a dose-dependent increase in the frequency of genetic duplications, and very high frequencies of duplications are recovered at high doses. In contrast, brucine, the 2,3-dimethoxy derivative of strychnine, caused no increase in duplication frequencies under the identical conditions.