Mono- and multiple-chlorinated propanones constitute a substantial portion of haloketones which have been identified as by-products of chlorination of humic substances or wood kraft pulp. Studies were conducted to compare the chemical reactivity and cytotoxicity of three representative chloropropanones: monochloropropanone (MCP), 1,1-dichloropropanone (1,1-DCP) and 1,3-dichloropropanone (1,3-DCP). It was found that chloropropanones reacted with the sulfhydryl, glutathione (GSH), without a metabolic activation system. The rate of reaction of chloropropanones with GSH in 0.1M phosphate buffer increased from pH 6 to 8 with the following order of reactivity: 1,3-DCP > MCP > 1,1-DCP. Biochemical studies with rat liver cytosol showed that MCP and 1,1-DCP were substrates for GSH S-transferase but enzymatic conjugation could not be determined for 1,3-DCP. All chloropropanones were cytotoxic in rat hepatocyte suspensions in the millimolar range with a concomitant decrease in cellular GSH. These results suggest that cytotoxicity produced by chloropropanones probably results from reaction with cellular sulfhydryl groups in which chemical reactivity and GSH conjugation of chloropropanones may determine relative toxicity of isomers.