Over the past twelve years, numerous reports have appeared in the literature documenting the presence of genotoxic activity in organic concentrates of drinking water. Genotoxic activity has been observed using in vitro test systems, including microbial, mammalian and lower eukaryotic cells and using whole plants. Although few studies have been performed to examine genotoxic damage in vivo, little or no evidence of genotoxicity has yet been observed using whole animal assay systems. A number of potential sources exist for the genotoxicity, but the chlorination stage of water treatment appears to be primarily responsible for the widespread occurrence of mutagens in drinking water.