3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) was detected by gas chromatography/mass spectrometry in drinking water samples from three locations in the United States, and also in a chlorinated humic acid solution. MX appears to account for a significant proportion of the mutagenicity of these samples, as measured in the Ames test using strain TA100 without metabolic activation. Studies on recovery of MX from spiked water samples by XAD-2/8 resin adsorption/acetone elution indicated that sample acidification prior to resin adsorption was essential to the effective recovery of MX. The stability of MX in aqueous solution was pH- and temperature-dependent. Overall, these results suggest that MX is formed during water chlorination as a result of reaction of chlorine with humic substances, and that a substantial fraction of the MX formed is likely to persist throughout the distribution system.