A study was designed to evaluate the effect of supplementation with a low dose (0.9 g/d) of n-3 polyunsaturated fatty acids (n-3 PUFA) in fish oil on the oxidative modification of low-density lipoprotein (LDL) in a group of healthy volunteers. Eight volunteers were randomly selected from a larger fish oil supplementation study, and were required to take either 0.9g n-3 PUFA as fish oil (FO group) or 0.9g olive oil [control oil (CO group)] for 16 weeks. Oxidative modification of LDL was assessed by measuring concentrations of foe cholesterol (FC), cholesteryl esters (CE) and cholesteryl linoleate hydroperoxide (Ch18:2-OOH) in LDL following copper-induced lipid peroxidation for 0, 2, 3 and 4 h. The composition of LDL fatty acids over 4 h of copper-induced oxidation was also investigated. LDL eicosapentaenoic acid (C20:5n-3) and docosahexaenoic acid (C22:6n-3) compositions were significantly (P < 0.05) higher in the FO compared with the CO group, following supplementation. Linoleic acid (C18:2n-6), arachidonic acid (C20:4n-6), C20:5n-3 and C22:6n-3 were significantly (P < 0.05) oxidised in LDL following 4 h copper-oxidation. The proportions of palmitic acid (C16:0) (P < 0.05), palmitoleic acid (C16:1) (P < 0.05), stearic acid (C18:0), and oleic acid (C18:1) increased in the FO and CO groups, after 4 h of copper-oxidation. Concentrations of cholesteryl oleate (Ch18:1), cholesteryl linoleate (Ch18:2n-6), cholesteryl arachidonate (Ch20:4n-6) and cholesteryl docosahexanoate (Ch22:6n-3) were significantly (P < 0.05) reduced following copper-stimulated oxidation, in both groups. Ch18:2-OOH concentrations were significantly increased (P < 0.05) following 3 h oxidation in both groups compared with 0 h copper-oxidation, but decreased after 4 h. There was no significant difference in concentrations of Ch18:2-OOH between the groups over the time-course of copper-mediated oxidation. The results of this study suggest that moderate dietary intakes of n-3 PUFA do not significantly influence the susceptibility of LDL to copper-induced oxidation in vitro. (C) 2000 Elsevier Science Inc.