Humic water was treated with different disinfectants and examined for mutagenicity, 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), E-2-chloro-3-(dichloromethyl)-4-oxobutenoic acid (E-MX), adsorbable halogenated organics (AOX), and chloroform. The highest mutagenic activity and the highest concentrations of MX and E-MX were noted for water disinfected with chlorine. An increased proportion of chlorine dioxide in the combined chlorine/chlorine dioxide treatment resulted in a decrease in the mutagenic activity and in the concentrations of MX, E-MX, AOX and chloroform. Ozone treatment prior to chlorination gave a slight decrease in mutagenicity and in the concentrations of MX and E-MX. Although the amount of AOX in monochloramine treated water was comparable to that in chlorinated water chloramination produced considerably lower mutagenicity and lower concentrations of MX and E-MX. MX was responsible for 20–50% and E-MX for a few percent of the mutagenic activity generated by the disinfectants.