In previously reported in vitro studies, we found that heme, a physiologically widespread hydrophobic iron compound, can rapidly generate oxidized low-density lipoprotein (LDL), which then becomes cytotoxic to cultured vascular endothelial cells; both LDL oxidation and endothelial cytotoxicity were inhibited by incubation with exogenous a-tocopherol (vitamin E) or ascorbic acid (vitamin C). Seeking relevance to in vivo conditions, we performed a study in which 10 human volunteers were given daily antioxidant supplements of 800 IU of DL-<x-tocopherol acetate alone or in combination with 1000 mg of ascorbic acid for 2 weeks. LDL resistance to heme oxidation ex vivo, as measured by the lag time for conjugated-diene formation, increased by as much as threefold from a mean±SD of 58±11 to 104±18 minutes (p<.001); LDL a-tocopherol increased from 11 ±2 to 26±6 molecules per LDL particle (p<.001); and most impressively, cytotoxicity to porcine aortic endothelial cells incubated with LDL conditioned with heme plus H2O2or with copper was completely prevented (cytotoxicity before supplementation was 42 + 12%, decreasing after supplementation to 3±2%,p<.001). These measurements reverted to their presupplement levels within 2 weeks after participants stopped taking antioxidant supplements and were reproduced in 4 subjects taking 800 IU of DL-a-tocopherol acetate supplements alone but not in the same subjects taking 1000 mg ascorbic acid supplements alone. In conclusion, oral vitamin E supplementation increases LDL or-tocopherol content, increases LDL resistance to oxidation, and decreases the cytotoxicity of oxidized LDL to cultured vascular endothelial cells.