The chlorinated hydrocarbons chloroform (CHCl3), 1,1-dichlorethane (1,1-DCE) and 1,2-dichloroethane (1,2-DCE) have been detected in finished drinking water. When administered to B6C3F1 mice by gavage in corn oil, these compounds have been shown to induce hepatic tumors. The present study examines the effect on liver tumor incidence of continuous treatment of CHCl3 (600 mg/L and 1800 mg/L), 1,1-DCE (835 mg/L and 2500 mg/L), and 1,2-DCE (835 mg/L and 2500 mg/L) administered in drinking water to male B6C3F1 mice using a two-stage (initiation/promotion) treatment protocol. Seventy 4-week-old male B6C3F1 mice constituted each treatment group. Of these mice, 35 were initiated by treatment with diethylnitrosamine (DENA) (10 mg/L) in the drinking water for 4 weeks. The remaining 35 received deionized drinking water. Each group was subsequently treated with one of two concentrations of CHCl3, 1,1-DCE, or 1,2-DCE in drinking water for 52 weeks. An additional group received phenobarbital (PB) (500 mg/L) and served as the positive control for liver tumor promotion. Mice were sampled after 24 weeks (10 mice) and 52 weeks (25 mice). At sampling, liver and lung tumors were detected. None of the compounds increased the number or incidence of lung or liver tumors by themselves. PB promoted liver tumor formation (but not lung tumors) in the DENA-initiated mice. 1,1-DCE and 1,2-DCE did not affect the incidence or number of liver or lung tumors in the DENA-initiated animals. CHCl3, however, inhibited liver and lung tumorigenesis in the DENA-initiated mice.