The genotoxic environmental pollutant 3-nitrofluoranthene (3-NFA) was reduced chemically and allowed to react with calf thymus DNA, yielding one major adduct which was determined to be N-(deoxyguanosin-8-yl)-3-amino-fluoranthene based on Fast Atom Bombardment Mass Spectrometry (FAB-MS), proton nuclear magnetic resonance, ultraviolet-visible wavelength light spectroscopy (UV-VIS), and fluorescence data. Extensive characterization of the isolated adduct by tandem mass spectrometry (MS/MS) was necessary to demonstrate definitively that the adduct isolated was the dG:C8 adduct, and not the isomeric dG:N2 adduct. The extent of modification of the initial calf thymus DNA by chemically reduced 3-NFA was 0.12% (1.2 adducts/10(3) nucleosides), which was sufficient to allow several hundred micrograms of the adduct to be isolated and purified. The chemically synthesized adduct was utilized as a reference standard for comparison to the major adduct isolated from xanthine-oxidase-catalyzed reduction of 3-NFA in vitro. The yield from the in vitro biological system was 2.4 adducts/10(5) nucleosides; the adduct isolated possessed the same mass spectrometric, UV-VIS, and fluorescence characteristics as the purified standard, and co-eluted with the standard on HPLC. No evidence for other adducts was found, either in vitro or in the chemical synthesis, based on FAB-MS examination of whole extracts of the reaction mixture for the presence of ions related to other possible adducts. Therefore, if minor adducts were present they were formed in substantially lesser amounts than N-(deoxyguanosin-8-yl)-3-aminofluoranthene.