The bifunctional aldehydes glyoxal and malondialdehyde, known to be toxic products of the radiolysis of sugar and other food constituents, interact with DNA in vitro and in vivo as demonstrated by thermal denaturation profiles, chromatographic behavior, and incomplete degradation of the reaction product by deoxyribonuclease. After irradiation of DNA in diluted radioactive glucose solution followed by incubation at slightly acid pH, the radioactivity is found in the core of the enzymic digest only. Attempts to locate the binding site proved that the radiolytic compound is associated preferentially with the guanine and cytidine moiety of the DNA core.