Drinking Water obtained from surface sources contains a very large variety of organic chemicals. The total organic carbon present in the source water is made up of both natural and man-made chemicals. In most instances natural organic material predominates and is largely made up of humic and fulvic acids. The introduction of chlorine into drinking water results in the formation of a variety of by-products including the trihalomethanes, haloacetonitriles, halogenated aldehyde and halogenated ketone derivatives. Representatives of these classes of chemicals have been shown to be mutagenic and/or carcinogenic. More recent studies have shown that similar chemicals are formed upon direct administration of chlorine solutions to rats. These data suggest that chemical interactions between a group of chemicals that have been generally regarded as safe (the disinfectants) and other chemicals of a low level of intrinsic toxicity (humic acids, stomach contents) produce potentially hazardous products.