Carotenoids and vitamin E are transported in human plasma complexed with lipoproteins. The bulk of them are associated with low-density lipoprotein (LDL), in which form they may act as antioxidants and thus delay the onset of atherosclerosis. We used a simple, rapid, ultracentrifugation technique to fractionate plasma lipoproteins in self-generating gradients of iodixanol (Optiprep), a non-ionic iodinated density gradient medium. The carotenoid content and composition of a number of LDL subfractions was determined by reversed-phase high-performance liquid chromatography. Lycopene, beta-carotene and beta-cryptoxanthin were mainly located in the larger, less-dense LDL particles whereas lutein and zeaxanthin were found preferentially in the smaller, more dense LDL particles. When the antioxidant content of these fractions was expressed per milligram of LDL protein, significantly lower concentrations of carotenoid and vitamin E were found to be associated with the smaller, protein-rich fractions of LDL. Strong positive correlations were found between total carotenoid and vitamin E plasma concentrations and the lag-time of Cu(2+)-mediated oxidation of LDL subfractions. The more dense LDL subfractions, which had lower levels of these antioxidants, were more readily oxidized, highlighting their possible role in atherosclerotic events.