Tamoxifen, a breast cancer drug, has recently been approved for the chemoprevention of this disease. However, tamoxifen causes hepatic carcinomas in rats through a genotoxic mechanism and increases the risk of endometrial tumors in women. DNA adducts have been detected at low levels in human endometrium, and there is much interest in determining whether DNA damage plays a role in tamoxifen-induced endometrial carcinogenesis. This study investigates the mutagenicity of tamoxifen DNA adducts formed by α-acetoxytamoxifen, a reactive ester producing the major DNA adduct formed in livers of tamoxifen-treated rats. pSP189 plasrnid DNA containing the supF gene was treated with α-acetoxytamoxifen and adduct levels (0.5-8.0 adducts per plasmid) determined by 32P-postlabeling. Adducted plasmids were transfected into nucleotide excision repair proficient (GM00637) or deficient (GM04429, XPA) human fibroblasts. After replication, plasmids were recovered and screened in indicator bacteria. Relative mutation frequencies increased with the adduct level, with 1.3-3.6-fold higher numbers of mutations in the XP cells compared to the GM00637 cells, indicating that NER plays a significant role in the removal of these particular tamoxifen DNA adducts. The majority of sequence alterations (91-96%) occurred at GC base pairs, as did mutation hotspots, although the type and position of mutations was cell-specific. In both cell lines, as the adduct level increased, the proportion of GC → AT transitions decreased and GC → TA transversions, mutations known to arise from the major tamoxifen adducts, increased. Given the high mutagenicity of dG-N2-tamoxifen adducts, if not excised, they may potentially contribute to the initiation of endometrial cancer in women.