Malondialdehyde (MDA), a product of lipid peroxidation, causes mutations in bacterial and mammalian cells and cancer in rats. MDA reacts with deoxynucleosides in vitro and the monomeric adduct of MDA with deoxyguanosine (M1G-dR) is the major adduct. M1G-dR has been detected in rat and human liver. Random mutagenesis studies with MDA-modified DNA and recent 32P-postlabeling studies indicate that in addition to M1G-dR, adducts to deoxyadenosine may also be formed. We have utilized liquid chromatography coupled with electrospray ionization tandem mass spectrometry to characterize an N6-oxopropenyl-2'-deoxyadenosine adduct (M1A-dR) in calf DNA modified with MDA.