A CHARACTERISTIC pink coloration was observed by Kohn and Liversedge1 after heating tissue suspensions with aqueous thiobarbituric acid (TBA). Colour formation was attributed to reaction of unknown oxidation products of unsaturated fatty acids2. The principal TBA-reactive substance is considered to be malonaldehyde3,4. However, the means of formation of malonaldehyde is unknown. Inasmuch as ultra-violet irradiation of unsaturated fatty acids had been used to produce TBA-reactive products5, investigation of the radiation-induced oxidation was conducted in this work in an attempt to clarify the basis of the TBA test.