Oxidized LDL exhibits cytotoxic activity towards endothelial cells, which is thought to be mediated by the products derived also from cholesterol oxidation, mainly involving the B-ring. By LC-PB-EI-MS and EI-MS/MS analysis, we have identified 3 beta-hydroxy-5 alpha-cholest-6-ene-5-hydroperoxide among the cholesterol oxidation products in LDL incubated with micromolar concentrations of copper ions.