Formation of mutagenic compounds as a result of aqueous chlorination of humic acids has been demonstrated. Solvent extracts responding positively to the Ames test were analyzed by GC/MS in an attempt to identify the mutagenic components. Aliquots of the chlorinated humic solution were also analyzed by closed-loop stripping and GC/MS. Results of GC/MS analyses of methylene chloride and ether extracts and of the closed-loop-stripping extract indicated that trihalomethanes and halogenated acetic acids, acetonitriles, 2-propanones, propenals, propenenitriles, propenes, phenols and thiophenes were formed by the aqueous chlorination of humic acids at neutral pH. The concentrations of eight of the halogenated by-products accounted for about one fourth of the total organic halogen (TOX) content of the aqueous chlorinated humic acid solution. The mass spectra of tentatively identified compounds are included. The similarity of mutagenic compounds identified in this study with compounds previously identified in drinking water suggests that the reaction of chlorine with natural aquatic humics is a likely source of mutagen formation in drinking water. Some data on the production of halogenated compounds from the chlorination of humics in the presence of bromide (Br-) at various molar concentrations are also included.