A method for the determination of hemoglobin (Hb) adducts of malonaldehyde (MA), one of the main products of lipid peroxidation, was developed. The method was based on a sodium borohydride reduction of MA-adducts to stable 3-hydroxypropyl adducts and quantification of the adduct to the N-terminal valine of Hb by a modified Edman procedure. Background levels of approximately 0.2 and 3.8 nmol/g Hb were recorded in one human volunteer and in control mice, respectively. Increased adduct levels were observed in mice after induction of lipid peroxidation with carbon tetrachloride. The kinetics of the reaction of MA with Hb and the stability of adducts formed were studied in in vitro experiments. Based on the adduct levels in Hb and the rate constant for adduct formation, the concentration of free MA in erythrocytes in mice was estimated. The background levels of adducts of some other aldehydes in Hb were also measured and the possibility that these adducts are formed as artefacts was studied.