In addition to its use in removal of compounds which cause taste, odor or color problems in drinking water, granular activated carbon (GAC) has received increasing attention as a treatment for removal of compounds known or suspected to be of toxicological significance. An experimental full scale GAC system for such water treatment has been developed at the Cincinnati Water Works. The authors have recently described the results of a study which examined this system throughout the adoptive life of the GAC, monitoring trends both in the production of nonvolatile mutagens by chlorine disinfection and in the removal of such bioactive compounds from the water. Mutagenicity values were determined with Salmonella tester strains in assays of water residue organics extracted using XAD-2. None of the samples of settled unchlorinated water was mutagenically active in that study, while such samples were consistently active following chlorination. Mutagenic activity in chlorinated water was effectively removed by GAC treatment throughout that study, even after the removal of total organic carbon (TOC) had reached a steady level of about 0.65 that contained in the water influent to the column. They then demonstrated that residues extracted from the used GAC were mutagenic. Differences in that activity were shown for residues recovered from the top, middle or bottom of the used CAG adsorber. This paper presents further analysis of such GAC extracted organics. The mutagens can be separated into two populations, one of which contains relatively non polar compounds and shows mutagenic activity similar to that seen among other residues from this drinking water source, whether extracted by us using XAD resins, or by others using reverse osmosis. In the other population more polar compounds predominate, including compounds and mutagenic activity which are poorly recovered using our XAD-2/XAD-7 resin procedure.