The oxidation of low density lipoprotein (LDL) in the arterial wall may contribute to atherogenesis. The oxidation of LDL by cells usually requires catalytically active transition metal ions. We show here some that gruel samples from human advanced atherosclerotic lesions are capable of catalysing the oxidation of LDL by macrophages as measured by thiobarbituric acid-reactive substances, enhanced electrophoretic mobility and increased macrophage uptake. This catalysis could be inhibited by pretreatment of the gruel with Chelex-100, which binds transition metal ions. The presence of catalytically active transition metal ions in atherosclerotic lesions may help to explain why LDL oxidation occurs at these sites.