The strong mutagen, [3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone] (MX) was found to occur in chlorinated drinking water in China. Some tap water samples from Jiangsu province, which contained MX of 3-60ng/L, were analyzed. Quantitation is based on gas chromatography-mass spectrometry, using peak area of selective ion monitoring of the methyl derivative of MX.