Malonaldehyde (malondialdehyde, MDA) was reacted with 2'-deoxyadenosine in buffered aqueous solution. HPLC analyses of the reaction mixtures showed that, besides the two previously characterized N6-propenal (M1dA) and N6-oxazocinyl (M3dA) adenine adducts, a third compound eluting at longer retention time was formed. The compound generated a strong peak in the chromatogram recorded by a fluorescence detector. The new compound was isolated by preparative C18 chromatography, and its structure was characterized by UV absorbance, fluorescence emission, 1H and 13C NMR spectroscopy, and mass spectrometry. The product was identified as 9-(2'-deoxyribosyl)-6-(3,5-diformyl-4-methyl-1, 4-dihydro-1-pyridyl)purine (M2AA-dA). The yield of the product was 0.8% following 7 days of reaction at 37 degreesC and pH 4.6. Lower yields were obtained at higher pH conditions. By the addition of acetaldehyde, the yield increased about 10-fold at all studied pH conditions. The adduct was most likely formed by an initial condensation of two molecules of malonaldehyde with one molecule of acetaldehyde followed by reaction of the condensation product with the exocyclic amino group of 2'-deoxyadenosine. The identification of this adduct shows that acetaldehyde may react with DNA bases also through an initially formed malonaldehyde-acetaldehyde condensation product.