A method for evaluating the mutagenic effects of a compound or drug, used as part of toxicity testing.

The test chemical is applied to a bacterial strain that has a mutation which renders it dependent on histidine for growth, along with a rat liver extract (to explore the potential for enzymatic processing of the chemical into a mutagenic form). After culture, the number of colonies that can grow in the absence of histidine, due to reversion of the mutation, provides a measure of the mutagenic potential of the chemical.

There is a reasonable degree of correlation between mutagenicity as judged by the Ames essay and carcinogenicity in animal models. However, it is a screening tool with limited predictability when used without other assays.