The organics in a Cincinnati, Ohio drinking water sample were concentrated by a reverse osmosis (RO) process. The diethyl ether soluble extract of the RO concentrate which proved to be mutagenic in studies using the Ames test, was partitioned into acid and base/neutral fractions. The unpartitioned ethyl ether concentrate, an acid and methylated acid fraction, the unpartitioned base/neutral extract, and five base/neutral eluants from a silica gel microcolumn were analyzed for the presence of organics using a computerized gas chromatography/mass spectrometry system (GC/MS/COM) equipped with glass capillary columns. Analysis of individual fractions indicated a predominance of polychlorinated biphenyls (PCBs) and chlorinated aromatics in the second base/neutral partition and many polynuclear aromatics (PNA) in the fourth base/neutral partition. Approximately 460 compounds were identified in this tap water extract, including 41 PNAs, 15 PCBs, and a number of amines, amides, and other halogenated species.