A highly sensitive and specific assay for the detection of N7-methyl-2′-deoxyguanosine (N7methyldG) has been developed by combining high-performance liquid chromatography, 32P-postlabeling, and nucleotide chromatography. Separation of normal nucleotides and adducts by high-performance liquid chromatography and then combining a portion of 2′-deoxyguanosine to the N7methyldG allows for quantitation using an internal standard. The directly determined molar ratio is not subject to errors in digestion, variable ATP-specific activity, or assumptions in relative adduct-labeling efficiency. The detection limit was one N7methyldG adduct in 107 unmodified 2′-deoxyguanosine bases. N7methyldG adducts have been detected in 5 human lung samples in which O6-methyl-2′-deoxyguanosine adducts had been previously determined. The mean ratio of N7methyldG to O6-methyl-2′-deoxyguanosine was determined to be approximately 10. The current assay complements the high-performance liquid chromatography/32P-postlabeling assay for O6-methyl-2′-deoxyguanosine and increases the detection sensitivity of DNA methylated by exogenous alkylating agents.