Diet and environmental exposures are often regarded as significant etiologic factors in human breast cancer. Chemicals that may be involved in these exposures include heterocyclic amines, aromatic amines, and polycyclic aromatic hydrocarbons, which also serve as strong mammary carcinogens in different animal models. In this study, we chose to quantify the major DNA adducts derived from one member of each of these classes of carcinogens, that is, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 4-aminobiphenyl (ABP), and benzo[a]pyrene (B[a]P), respectively, in DNA isolated from exfoliated ductal epithelial cells in human breast milk. Milk was collected from healthy, nonsmoking mothers. The isolated DNA was digested to 3' nucleotides and subjected to (32)P-postlabeling. Adduct enrichment was achieved using Oasis Sep-Paks and the analyses were conducted by HPLC using radiometric detection. Critical to the analyses were the syntheses of bis(phosphate) standards for the C8-dG adducts of PhIP and ABP, and the N(2)-dG adduct of B[a]P, which were added to each reaction as UV markers. Of the 64 samples analyzed, adducts were found in 31 samples. Thirty samples contained detectable levels of PhIP adducts, with a mean value of 4.7 adducts/10(7) nucleotides; 18 were positive for ABP adducts with a mean value of 4.7 adducts/10(7) nucleotides; and 13 were found to contain B[a]P adducts with a mean level of 1.9 adducts/10(7) nucleotides. These data indicate that women are exposed to several classes of dietary and environmental carcinogens and that these carcinogens react with DNA in breast ductal epithelial cells, the cells from which most breast cancers arise.