In this article, the authors report results of a pilot-scale investigation of enhanced coagulation for the removal of particles and trihalomethane (THM) precursors from two natural water sources, Colorado River water (CRW) and California State Project water (SPW). These water sources are used by the Metropolitan Water District of Southern California (MWDSC) to serve nearly 15 million people in a six-county area of Southern California. Samples of both source water and filter effluent were examined by counting particles and measuring particle size distribution, turbidity, total organic carbon, ultraviolet light absorbance at 254 nm (UV254), and THM formation potential. The implications of the results for current and pending regulations pertaining to particles and disinfection by-products (DBPs) are discussed. Includes 31 references, tables, figures.