Low-density lipoprotein (LDL) oxidation and macrophage cholesterol accumulation are both involved in atherogenesis. Recently it was shown that feeding rabbits or humans with an oleic-acid-rich diet reduced the susceptibility of their LDL to in vitro oxidation. Since olive oil is highly enriched with oleic acid, we tested the effect of olive oil supplementation (50 g/day) to the diet of 10 healthy male subjects, during a 2-week period, on macrophage uptake of their LDL and on the propensity of their LDL to oxidation (with copper ions). Olive oil supplementation to the diet resulted in LDL enrichment with oleic acid (C18:1) and sitosterol. No effect on plasma cholesterol levels was found, but the LDL cholesteryl ester content was reduced (16%) whereas its unesterified cholesterol was increased (41%). Even after 1 week of the olive oil diet, the LDL susceptibility to in vitro oxidation was significantly reduced (p < 0.01). Macrophage uptake of LDL was studied by analysis of cellular cholesterol content and by analysis of the macrophage cholesterol esterification rates. LDL obtained after 1 and 2 weeks of the olive oil diet demonstrated reduced cellular uptake in comparison with LDL obtained before the supplementation of olive oil, by 50 and 61%, respectively. The LDL resistance to oxidation was shown by a reduction in its peroxide, malondialdehyde and conjugated diene content by 73, 28 and 32%, respectively. LDL incubation with oleic acid for the period of its oxidation with copper ions demonstrated a dose-dependent inhibition of lipoprotein oxidation by up to 72% as opposed to linoleic and arachidonic acids (50 microM) which increased LDL oxidation by 22 and 72%, respectively. Sitosterol, in a similar incubation system, inhibited LDL oxidation by up to 26%. We conclude that olive oil supplementation to the diet modifies LDL lipid composition and enriches the lipoprotein with oleic acid and sitosterol. The antiatherogenic properties of this modified lipoprotein may be related to its resistance to in vitro peroxidation and its reduced uptake by macrophages.