A comparison has been made between the peroxidation rate as a result of ionizing radiation in liposomes prepared from phospholipids which were extracted from biological membranes, in single component micelles and in micelles of mixed composition. The ease of fatty acid oxidation in the different preparations was studied at a variety of pH values. The damage has been quantified spectrophotometrically in terms of diene conjugation (233 nm) and as the disappearance of fatty acids by gas chromatography. The ease of fatty acid oxidation was in the following order for the liposomal and mixed micelle preparations: 22:6 greater than 20:4 greater than 18:2. For single component micelles the order was reversed: 18:2 greater than 18:3 greater than 20:4 greater than 22:6. The micellar lipid preparations were pH-dependent in their response to radiation, which was demonstrated by a dip in the pH-response curve. Peroxidation of especially 22:6 was promoted when present in mixed micelles with 18:2.