An in vivo system has been devised in which chemical mutagen-carcinogens injected into an animal induce sister chromatid exchanges that can be observed when the animal's peripheral lymphocytes are subsequently cultured and then stained with the FPG technique. Chemicals requiring metabolic activation, as well as those that do not, produce significant increases in SCE frequency one day after exposure. The frequency then returns to control level within two weeks. This in vivo system is a highly sensitive assay for the ability of chemical agents to damage chromosomes.