The reaction of the alpha, beta-unsaturated ketones methyl vinyl ketone (MVK) and ethyl vinyl ketone (EVK) with nucleosides and 5'-mononucleotides was studied. The genotoxic activity of MVK and EVK in the SOS Chromotest was investigated. Three different types of adducts with deoxyguanosine were found and their structures elucidated: the cyclic 1,N2 adducts, the linear N7 adducts with one still-unreacted carbonyl function, and the cyclic 1,N2, linear N7, bis adducts. The spectroscopic and other relevant characterization data for the deoxyguanosine adducts and the corresponding guanine adducts are presented here together with details of the chromatographic methods used for isolation. The adducts described could also be isolated in the reactions of MVK and EVK with 2'-deoxyguanosine 5'-monophosphate. No adducts could be isolated either with nucleosides other than deoxyguanosine or with nucleotides other than 2'-deoxyguanosine 5'-monophosphate, indicating that the guanine moiety is the most reactive DNA constituent for MVK and EVK. MVK and EVK were clearly genotoxic in the SOS Chromotest according to the criteria of Quillardet and Hofnung. The formation of these adducts was proposed as the mechanism for the genotoxicity of MVK and EVK: all data available support the assumption that MVK and EVK represent a mutagenic and carcinogenic risk for mankind.