The effect of beta-carotene on the susceptibility of low density lipoprotein (LDL) to oxidative modification was investigated in a double-blind, randomized placebo-controlled study. Hypercholesterolaemic, postmenopausal women were given 30 mg beta-carotene per day (n = 15 subjects) or placebo capsules (n = 15 subjects) for 10 weeks. They were instructed to follow the American Heart Association Step One diet. LDL, isolated before and after treatment was subjected to copper-catalysed lipid peroxidation. There were no significant differences between LDL from the beta-carotene and placebo groups, as assessed by measuring the lag time for formation of conjugated dienes; the rate of formation and the amount of conjugated dienes formed; the amount of lipid peroxides generated; and the relative electrophoretic mobility, at baseline and after treatment. Dietary records showed that the subjects were consuming similar amounts and types of fat. No significant differences were found in the lipid composition and fatty acid pattern of LDL from the two groups. In conclusion, the results indicated that supplementation with beta-carotene in non-smoking, hypercholesterolaemic, postmenopausal women had no protective effect on the susceptibility of LDL to copper-catalysed modification in vitro.