Lifetime ingestion by rats of 0.2 mg/kg per day of hexachlorobutadiene [HCBD] caused no discernible ill effects. Slight degrees of primarily renal toxicity occurred with 2 mg/kg per day; 20 mg/kg per day caused multiple toxic effects, including renal tubular neoplasms. HCBD is a by-product of certain processes associated with the chlorination of hydrocarbons. Very small amounts of HCBD may also be produced in chlorine plants.