The free radical scavenging abilities of the structurally related steroids beta-sitosterol, beta-sitosterol glucoside (plant sterols and sterolins), cholesterol, and dehydroepiandrosterone sulphate (DHEAS) were compared with melatonin (an efficient free radical scavenger) in an in vitro system which measures lipid peroxidation of platelet membranes in the presence of iron (Fe2+). Lipid peroxidation is a process whereby cellular membranes are damaged due to the oxidative deterioration of polyunsaturated lipids, which may lead to cell death and disease in living organisms. Substances such as vitamin E protect cellular membranes against oxidative damage due to their chemical structures. The steroids cholesterol, beta-sitosterol, beta-sitosterol glucoside and dehydroepiandrosterone (DHEA) are structurally related to each other. During aging, serum concentrations of DHEA, DHEAS and melatonin decrease, while the concentration of cholesterol tends to increase. The aim of the present study was to compare the role these substances play in lipid peroxidation over a wide concentration range. At concentrations lower than the free iron in the reaction mixture, all the steroids investigated decreased lipid peroxidation. At higher concentrations, cholesterol and beta-sitosterol increased lipid peroxidation, while DHEAS and melatonin continued to decrease lipid peroxidation.