Byproducts of the chlorination of treated water, humic acids, and amino acids were shown to be mutagenic to Salmonella typhimurium strain TA100 in the fluctuation test. A two-stage fractionation procedure, using high-performance liquid chromatography (HPLC) combined with the fluctuation test, was developed. Several mutagenic fractions were obtained from an XAD extract of chlorinated water; this result indicated the presence of several mutagens. The mutagenic activities of extracts of different chlorinated amino acids were compared. The products identified by capillary gas chromatography-mass spectrometry did not account for the mutagenic activity. However, additional products were detected by HPLC with UV detection in extracts of chlorinated tyrosine and phenylalanine. (1-3). Compounds that are widely distributed in the aquatic environment, such as humic substances and amino acids, have been found to react with chlorine to produce mutagenic compounds (4, 5). These compounds may account for at least some of the mutagenic activity of extracts of drinking waters. CONCENTRATED EXTRACTS OF MANY DRINKING WATERS in the United Kingdom and elsewhere are mutagenic as observed in bacterial assaysBecause a good correlation is found between mutagenicity (as detected in bacterial assays such as the Ames test) and carcinogenicity (as observed in laboratory animal tests) (6), mutagenicity detected in water extracts indicates the possibility of a carcinogenic hazard.