The aim of this work was to compare the behaviour of HDL oxidised by .OH or .OH/O2.- free radicals produced by gamma radiolysis in the absence or in the presence of LDL at the same concentration of 3 g x l(-1), in order to specify the possibility of reciprocal protection of HDL and LDL towards lipid peroxidation. This oxidation was quantitatively evaluated by the decrease of endogenous alpha-tocopherol and the formation of oxidation products (thiobarbituric acid-reactive substances and conjugated dienes) and by the determination of initial radiation yields. Our results demonstrated that HDL could be protected by LDL against in vitro radical oxidation only in the presence of oxygen (action of .OH/O2.- free radicals). This observation addresses new questions about the interaction between HDL and LDL, especially the possibility of a reciprocal protection.