Humic acid chlorination products are being studied in an effort to identify the chemicals responsible for the mutagenicity formed during water chlorination. In the present report, nineteen chlorinated organic compounds have been identified and quantified in ether extracts of chlorinated humic acid solutions. Ten of these compounds, including a number of chlorinated propanones and chlorinated propenals, are direct-acting mutagens in the Salmonella/microsome mutagenicity assay. The position of the chlorine substituent has been found to be an important factor in the mutagenic activity of these two classes of compounds. The total mutagenicity of the compounds identified thus far, when tested either individually or as a composite, accounts for only about 8% of the total mutagenicity formed during humic acid chlorination. The addition of bromide to the humic acid chlorination reaction results in up to a two-fold increase in the level of mutagenicity formed.