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Hypocholesterolaemia in infantile Refsum disease (IRD) may link peroxisomes and lipoprotein metabolism. In our patient, plasma cholesterol levels were reduced to 26% and 29% of control in LDL and HDL fractions, respectively. Plasma apolipoproteins B-100 and A-I levels were 52% and 66% of controls, respectively. In the kindred, plasma cholesterol concentration was 61-73% of controls. The HDL-cholesterol/apo A-I ratios were: patient 0.12; kindred 0.17; controls 0.28. Analysis of the IRD patient's lipoprotein revealed compositional abnormalities in all fractions. The patient's LDL demonstrated a substantial reduction in its lipid-to-protein ratio. Alterations in plasma lipoproteins affect their interaction with macrophages. Upon incubation of the patient's LDL with J-774 macrophages, its cellular uptake, measured as cholesterol esterification rate, was only 66% of a control rate. The abnormal LDL of the IRD patient showed also only 25% of control susceptibility to in vitro oxidation. Studies of cellular cholesterol metabolism in the patient's monocyte-derived macrophages (MDM) showed 57% increased cholesterol esterification rate in comparison to normal MDM. The possible link between lipoprotein abnormalities and monocyte-macrophage cholesterol metabolism is discussed.