A test to determine the effects of a chemical on the rate of mutation in bacterial cells, and hence its likely potential for causing cancer in other organisms, including humans. Devised by US biologist Bruce Ames (1928– ), it is widely used in screening chemicals occurring in the environment for possible carcinogenic activity (see xenobiotic). The chemical is applied to plates inoculated with a special mutant strain of bacteria, usually Salmonella typhimurium, that require the amino acid histidine for growth. Cells that mutate back to the wild type are detected by the occurrence of colonies able to synthesize their own histidine and therefore to grow on the medium.