Hexachlorobenzene (HCB) and other chlorinated benzenes have had only limited mutagenicity evaluation but tests have shown a general lack of evidence for mutagenicity. Bacterial reverse mutation studies have been typically negative and only a few studies on cultured mammalian cells have been published. Recent reports indicate that HCB does not induce sister chromatid exchange or DNA-strand breakage in vitro. A single report of HCB-induced reverse mutation in yeast is suspect due to the failure to establish either dose-response effects or an acceptable level of statistical significance. In-vivo data are limited primarily to dominant lethal studies in rats and some in-vivo alkaline elution results. Both types of tests revealed no genetic activity for HCB. A number of chlorine-substituted nitrobenzenes are positive in the Ames Salmonella reverse mutation assay, but when tested in eukaryotic cell assays, the results are generally negative. Ortho-, meta- and para-dichlorobenzenes are apparently mutagenic in Aspergillus nidulans, and para-dichlorobenzene has been reported to induce mitotic abnormalities in plant cells. Unscheduled DNA synthesis studies carried out with dichlorobenzenes proved negative. Monochlorobenzene failed to actively induce sister chromatid exchange in a human cell line. The only human data that have been reported for chlorinated benzenes involved accidental exposure of 26 people to ortho-dichlorobenzene vapours. Analysis of lymphocytes from this cohort suggested an increase in chromosome breakage.