Disinfection of drinking water in the United States is accomplished by four major disinfectants: chlorine, monochloramine, chlorine dioxide, and ozone. These four disinfectants were applied to four parallel streams in a pilot plant located at Jefferson Parish, Louisiana. Several organics, including surrogates such as TOC and TOX, were evaluated to investigate the effects of disinfection and treatment by sand filtration and GAC adsorption. Also, five toxicological tests were conducted to determine the general toxicity and the mutagenic/carcinogenic potential of disinfection and/or GAC adsorption. Results of the study showed that ozonation produced less organics in most but not all cases. The short term animal toxicological studies revealed difficulties in analyzing actual drinking waters for detectable toxic effects.