The integrity and the surface charge distribution of native low density lipoprotein (LDL) are prerequisites of its binding to the LDL receptor. Oxidation is one of the most important physiological effects resulting in an altered structure and metabolism of LDL. To reveal forces responsible for maintaining the intact structure of LDL in the absence of cells we have determined the kinetics of lipid peroxidation, changes in electrophoretic mobilities and size distributions of LDL samples as a function of Cu++ induced oxidative modification in a cell-free system at two different (50 mM and 150 mM) ionic strengths by electrophoretic and dynamic light scattering. Our data show that the lipid peroxidation is almost complete before LDL is degraded at 50 mM while a slight extent of lipid peroxidation is enough to result in the same effect at 150 mM. These suggest that both ionic and hydrophobic interactions are necessary to maintain the integrity of the LDL molecule.