The membrane interactions of malonyldialdehyde (MDA), natural product of polyunsaturated fatty acids peroxidation were investigated by differential scanning calorimetry, and ESR or NMR spectroscopy. This component is located in the superficial part of the bilayer, where it increases the local fluidity. High concentrations of MDA induce major membrane damage. Similar consequences of MDA-membrane interactions were observed on erythrocyte ghosts.