A potent mutagen, 3-chloro-4-(dichloromethyl)-5-hydroxy2(5H)-furanone (MX) has been identified in three chlorinated drinking waters in the United States. It has also been identified in two chlorinated drinking waters by investigators in Finland. MX appears to account for a significant proportion of the total mutagenicity of these samples, as measured in the Ames test using strain TA100 without metabolic activation. Investigations into the products of chlorination of humic acids, which occur naturally in drinking water, indicate that this is the probable source of MX in chlorinated waters. The Health Effects Research Laboratory (HERL) has analyzed drinking water samples for MX by first concentrating organics using XAD resins, isolating a strong acid fraction using extraction techniques, and fractionating the strong acids using HPLC. HPLC fractions containing MX were analyzed by the Ames test and by capillary column GC/MS. Mutagenicity levels determined by the Ames assay were correlated with the amount of MX present as quantified by GC/MS. The data presented here show the reproducibility of the method and the average recovery of MX from spiked drinking water concentrates.