Recent studies have shown that the compound MX [3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone] can account for up to 57% of the observed acid fraction mutagenicity of chlorinated drinking water. In the present study laboratory investigations showed MX to be very well removed by activated carbon over a wide concentration range. Samples obtained from two water treatment plants following pre-chlorination showed MX to be present in the range of 38 to 60 ng L -1 . MX concentrations were not always correlated with TOC levels, suggesting that the type of organic matter present may play a role in MX formation. GAC appeared to be capable of removing MX precursors.