Halogenated acetonitriles and acetones are common by-products of drinking water chlorination and have been identified in reactions with model substrates. Members of both classes have been shown to be mutagenic in Salmonella, to induce sister chromatic exchange in mammalian cells and/or to interact with DNA. Such results would imply a capability for these chemicals to act as carcinogens. The present study was undertaken to determine whether the members of these classes most often found in drinking water were capable of acting as carcinogens on the mouse skin or lung. Studies included the chloro-, dichloro-, trichloro-, bromochloro- and dibromo- substituted acetonitriles. The 1,1-dichloro- and 1,1,1-trichloro- substituted acetones were tested. These data clearly show that the haloacetonitriles found in drinking water, with the possible exception of the dichloro-compound possess carcinogenic properties. To date, it has not been possible to demonstrate such activity with the chlorinated acetones. These findings emphasize the need to consider by-products other than the trihalomethanes when assessing carcinogenic hazards that are associated with alternative forms of drinking water disinfection.