Malondialdehyde (MDA), an endogenous product of lipid peroxidation and prostaglandin biosynthesis, is mutagenic in bacterial and mammalian cells and carcinogenic in rats. In order to determine whether MDA-modified bases are formed in nucleic acids in vivo, sensitive immunoassays to detect MDA-DNA and MDA-RNA adducts are being developed in our laboratory. Murine monoclonal antibodies reactive with the MDA-deoxyguanosine adduct 3-β-D-erythro- pentofuranosylpyrimido[1,2-a]purin-10(3H)-one (M1G-R) were prepared and characterized. Several MDA-modified nucleosides and deoxynucleosides and structural analogs were synthesized and characterized and were compared as competitive inhibitors in enzyme-linked immunosorbent assays (ELISAs). Less than 5 fmol of M1G in MDA-modified DNA was detected in a direct ELISA, and antibody binding to the modified DNA was competitively inhibited by free M1G-dR. DNA from Salmonella typhimurium treated with concentrations of MDA that induce reversion to histidine prototrophy was enzymatically digested, and M1G-dR was quantitated by competitive ELISA. Over a range of MDA concentrations from 10 to 40 mM, the level of M1G residues in bacterial DNA increased from 0.2 to 2.5/106 base pairs.