A series of 15 halogenated hydrocarbons of industrial and environmental importance were tested for carcinogenicity by chronic administration by one or more routes in Ha:ICR Swiss mice. Not all compounds were tested by the four routes of administration used. Allyl chloride, 1,2-dibromo-3-chloropropane, and vinylidene chloride were active as skin tumor initiators in the two-stage carcinogenesis assays; phorbol myristate acetate was used as a promoter. 1,2-Dibromoethane was the only compound that induced a significant incidence (p less than 0.05) of skin papillomas, skin carcinomas, and lung tumors by repeated skin application. 1,2-Dichloroethane, 1,1,2,2-tetrabromoethane, and 1,2-dibromo-3-chloropropane induced lung and/or stomach tumors by repeated skin application. Two compounds showed sarcomagenic activity by sc injection; they were cis-1,3-dichloropropene and 2-chloropropanal. By intragastric intubation, 1-chloropropene and 2-chloropropanal induced significant numbers of stomach tumors. Vehicle, no-treatment, and positive control groups were included in these tests. The following compounds were also tested by one or more of the four routes but were inactive by the criteria used; i.e., they showed P = 0.05 or greater than 0.05: trichloroethylene, tetrachloroethylene, hexachlorobutadiene, chloroacetaldehyde, 1-chloropropene oxide (cis and trans), and trichloroethylene oxide.