Hexachlorobenzene (HCB) has been used as a fungicide, is a contaminant of various pesticides and is a by-product in the manufacture of many other chlorinated hydrocarbons. In Turkey, HCB caused an epidemic of toxic porphyria involving several thousand people between 1955 and 1959. The aim of the present studies was to determine the chronic toxicity of HCB after prolonged oral administration and we report here that HCB is carcinogenic in mice and hamsters. Mice and hamsters were given dietary HCB at doses of 50, 100 and 200 ppm for life, with deaths due to toxic manifestations of HCB occurring in group HCB 200. All survivors were killed after 120 weeks. In both mice and hamsters, exposure to HCB increased the incidence of liver-cell tumours but no such tumours were observed in the controls. A significant increase in the incidence of thyroid tumours and liver haemangioendotheliomas was observed in hamsters treated with HCB. Our findings that HCB is carcinogenic in mice and hamsters, together with other results showing the same effect in rats, provide sufficient experimental evidence to recommend great caution in the use of this material.