This part of ISO 21427 specifies a method for the determination of genotoxicity of water and waste water using a mammalian in vitro test which detects damage, induced by water-soluble substances, to the chromosomes or the mitotic apparatus of V79 cells from the Chinese hamster. The micronucleus test allows the identification of substances that cause cytogenetic damage which results in the formation of micronuclei containing lagging chromosome fragments and/or whole chromosomes. The assay is based on the increase in the frequency of micronucleated cells after incubation with and without metabolic activation.