The etiology of human breast cancer is poorly understood, but circumstantial evidence points toward exogenous genotoxins as causative agents; they are believed to exert their carcinogenic action by binding to DNA. Because this binding is often preceded by metabolic activation, it is dependent on the expression and activities of metabolic enzymes of the host. Human mammary tissue samples from 42 women undergoing surgery for breast cancer or reduction mammoplasty were analyzed for DNA adducts by 32P-postlabeling analysis. With the butanol extraction method of DNA adduct enrichment, adduct levels were determined to be 0-414.6 adducts per 10(9) nucleotides, with considerable interindividual variation. To characterize the DNA adducts, we reanalyzed the adduct spots by reversed-phase high-performance liquid chromatography. Of two major adduct spots detected on TLC that accounted for up to 70% of the DNA modification, one eluted as a single peak on high-performance liquid chromatography, whereas the other was resolved into two distinct peaks of radioactivity. These major adducts were highly lipophilic in character. The N-acetyltransferase-1 (NAT1) and NAT2 genes were analyzed for common mutations using random RFLP analysis. An association between NAT2 acetylator status and adduct levels was observed; significantly elevated adduct levels occurred in the mammary DNA from women who were designated slow acetylators for NAT2 [median adduct level = 83.0 adducts per 10(9) nucleotides (range, 9.0-414.6)], as compared with the levels in individuals designated rapid acetylators for NAT2 [median adduct level = 39.7 adducts per 10(9) nucleotides (range, 0-91.0; P = 0.0053)]. On the other hand, NAT1 genotypes were not significantly associated with adduct levels. Although the agents responsible for the DNA modifications in the human breast are not known, this pilot study supports the hypothesis that DNA adduct formation in the human breast may be influenced by the NAT2 genotype.