This Test Guideline describes an in vivo assay that detects chemicals that may induce gene mutations. In this assay, transgenic rats or mice that contain multiple copies of chromosomally integrated plasmid or phage shuttle vectors are used. The transgenes contain reporter genes for the detection of various types of mutations induced by test substances. A negative control group and a minimum of 3 treatment groups of transgenic animals are treated for 28 consecutive days. Administration is usually followed by a 3-day period of time, prior to sacrifice, during which the agent is not administered and during which unrepaired DNA lesions are fixed into stable mutations. At the end of this 3-day period, the animals are sacrificed, genomic DNA is isolated from the tissue(s) of interest and purified. Mutations that have arisen during treatment are scored by recovering the transgene and analysing the phenotype of the reporter gene in a bacterial host deficient for the reporter gene. Mutant frequency, the reported parameter in these assays, is calculated by dividing the number of plaques/plasmids containing mutations in the transgene by the total number of plaques/plasmids recovered from the same DNA sample.