Bromodichloromethane is a by-product of the chlorination of drinking water. It is formed by the halogen substitution and oxidation reactions of chlorine with naturally occurring organic matter (e.g., humic or fulvic acids) in water containing bromide. Bromodichloromethane has been shown to be carcinogenic at multiple sites in rats (large intestine and kidney) and in mice (liver and kidney) after administration by gavage in corn oil. To further characterize its dose-response relationships for evaluations of human risk, bromodichloromethane was nominated to the NTP by the United States Environmental Protection Agency for toxicity and carcinogenicity studies in rats and mice by drinking water exposure. Male F344/N rats and female B6C3F1 mice were exposed to bromodichloromethane (greater than 98% pure) in drinking water for 3 weeks or 2 years. Genetic toxicology studies were conducted in Salmonella typhimurium, L5178Y mouse lymphoma cells, cultured Chinese hamster ovary cells, mouse bone marrow cells, and mouse peripheral blood erythrocytes. 3-WEEK STUDY IN RATS: Groups of 10 male F344/N rats were exposed to target concentrations of 0, 43.7, 87.5, 175, 350, or 700 mg/L bromodichloromethane (equivalent to average daily doses of approximately 0, 6, 12, 20, 38, or 71 mg bromodichloromethane/kg body weight) in drinking water for 3 weeks. All rats survived to the end of the study. The mean body weight gains of 350 and 700 mg/L rats were significantly less than that of the controls. Concentration-related decreases in water consumption were evident during the first week on study. Relative kidney weights of rats in the 175, 350, and 700 mg/L groups were significantly greater than that of the controls. There were no significant chemical-related histopathological changes. 3-WEEK STUDY IN MICE: Groups of 10 female B6C3F1 mice were exposed to target concentrations of 0, 43.7, 87.5, 175, 350, or 700 mg/L bromodichloromethane (equivalent to average daily doses of approximately 0, 6, 10, 16, 29 or 51 mg/kg) in drinking water for 3 weeks. All mice survived to the end of the study. Final mean body weights of the 175, 350, and 700 mg/L mice and mean body weight gains of 350 and 700 mg/L mice were significantly less than those of the controls. These decreases were attributed to decreased water consumption. There were significant concentration-related decreases in water consumption by groups exposed to 87.5 mg/L or greater throughout the study; these decreases were attributed to poor palatability of the dosed water. Relative liver, kidney, and thymus weights of mice in the 350 and 700 mg/L groups were significantly greater than those of the controls. Absolute lung weights of mice in the 350 and 750 mg/L groups were significantly less than that of the controls. There were no significant chemical-related histopathological changes. 2-YEAR STUDY IN RATS: Groups of 50 male F344/N rats were exposed to target concentrations of 0, 175, 350, or 700 mg/L bromodichloromethane (equivalent to average daily doses of approximately 0, 6, 12, or 25 mg/kg) in drinking water for 2 years. Survival of exposed groups was similar to that of the controls. Mean body weights of all exposed groups were generally similar to those of the controls throughout the study. Water consumption by exposed rats was less than that by the controls throughout the study; the decreases were attributed to poor palatability of the dosed water. There were no increased incidences of neoplasms that were attributed to bromodichloromethane. The incidences of chronic inflammation in the liver of the 350 and 700 mg/L groups were significantly greater than that in the controls; however, the biological significance of these increases is uncertain. 2-YEAR STUDY IN MICE: Groups of 50 female B6C3F1 mice were exposed to target concentrations of 0, 175, 350, 700 mg/L bromodichloromethane (equivalent to average daily doses of approximately 9, 18, or 36 mg/kg) in drinking water for 2 years. Survival of exposed groups was similar to that of the controls. Mean body weights of all exposed groups were generally less than those of the controls from week 4 through the end of the study. Water consumption by exposed mice was less than that by the controls throughout the study; the decreases were attributed to poor palatability of the dosed water. The incidences of hepatocellular adenoma or carcinoma (combined) occurred with a negative trend, and the incidence in the 700 mg/L group was significantly decreased relative to the control group. The incidence of hemangiosarcoma in all organs was significantly decreased in the 350 mg/L group. RESULTS The results of in vitro mutagenicity tests with bromodichloromethane were mixed. Bromodichloromethane did not induce mutations in any of several tester strains of Salmonella typhimurium, with or without exogenous metabolic activation (S9 liver enzymes). In contrast to the negative results in Salmonella, tests for mutation induction in mouse lymphoma L5178Y/tk(+/-)cells were positive in the presence of induced rat liver S9; no mutagenic activity occurred in tests conducted without S9. In cytogenetic tests with cultured Chinese hamster ovary cells, bromodichloromethane induced a small increase in sister chromatid exchanges (SCEs) in one of four trials conducted in the presence of induced rat liver S9 enzymes; no significant increase in SCEs occurred without S9, and no induction of chromosomal aberrations occurred in bromodichloromethane-treated Chinese hamster ovary cells with or without S9. Results of in vivo tests for chromosomal damage were negative. No increases in the frequency of micronucleated erythrocytes were seen in bone marrow of male B6C3F1 mice administered bromodichloromethane by intraperitoneal injection for 3 days. In addition, no induction of micronuclei was observed in circulating erythrocytes of female B6C3F1 mice administered up to 700 mg/L bromodichloromethane in drinking water for 3 weeks. CONCLUSIONS Under the conditions of this 2-year drinking water study, there was no evidence of carcinogenic activity of bromodichloromethane in male F344/N rats exposed to target concentrations of 175, 350, or 700 mg/L. There was no evidence of carcinogenic activity of bromodichloromethane in female B6C3F1 mice exposed to target concentrations of 175, 350, or 700 mg/L.