The use of chlorine as a drinking water disinfectant is known to produce a variety of chlorinated and oxidized derivatives of organic matter present in the source water. Humic substances, which represent the major fraction of the organic matter in natural waters, have been shown to give rise to numerous carcinogenic and mutagenic substances upon chlorination. Attempts to fully characterize and/or identify the mutagenic reaction products were made using various isolation/concentration techniques in combination with GC/MS. Halogenated aldehydes, acids, ketones, nitriles, alkenes, and aromatic compounds were identified. The similarity of mutagenic compounds identified in chlorinated drinking water and in chlorinated solutions of commercial humic acid suggests that the reaction of chlorine with natural aquatic humic material is a likely source of mutagen formation in drinking water. While much of the mutagenic activity could be extracted, attempts to recover a significant portion of the activity from gas chromatographic columns were unsuccessful. Additional studies were conducted to determine the potential for the in vivo formation of halogenated or oxidized derivatives of endogenous organic material resulting from the ingestion of residual chlorine in potable water. GC/MS analyses of extracts of stomach contents and plasma, taken from fasted and non-fasted Sprague-Dawley rats dosed with NaOCL by gavage, demonstrated the in vivo formation of di- and trichloroacetic acids. Chloroform was identified in the stomach contents of the fasted and nonfasted animals dosed with NaOCL but was not detected in the control groups.