We investigated the effect of incorporating n-3 polyunsaturated fatty acids (PUFAs) into the diet on the lipid-class composition of LDLs, their size, and their susceptibility to oxidation. Forty-seven healthy volunteers incorporated 30 g sunflower-oil (SO) margarine/d into their habitual diet during a 3-wk run-in period and then used either SO or a fish-oil-enriched sunflower oil (FO) margarine for the following 4 wk. Plasma concentrations of total cholesterol, triacylglycerols, HDL cholesterol, LDL cholesterol, and apolipoproteins A-I and B did not differ significantly between the groups during intervention. The FO margarine increased the concentration of n-3 very-long-chain PUFAs in the LDL particles, showing 93% (P < or = 0.0001), 8% (P = 0.05), and 35% (P = < 0.0001) increases in eicosapentaenoic acid, docosapentaenoic acid, and docosahexaenoic acid, respectively, in the FO group compared with 3%, 7%, and 7%, respectively, in the SO group during the intervention. The cholesterol content of the LDL particles increased in the FO group [total cholesterol: 6% (P = 0.008); cholesterol ester: 12% (P = 0.014)], although it was not significantly different from that in the control group, whereas the other lipid classes and the size of the LDL particles remained unchanged in both groups. A reduction in the alpha-tocopherol content in LDL (6%, P = 0.005) was observed in the FO group. Ex vivo oxidation of LDL induced with Cu2+ showed a significantly reduced lag time (from 91 to 86 min, P = 0.003) and lower maximum rate of oxidation (from 10.5 to 10.2 nmol x mg(-1) x min(-1), P = 0.003) after intake of the FO margarine. The results indicate that consumption of the FO compared with the SO margarine had no effect on LDL size and lipid composition and led to minor changes in LDL a-tocopherol content and oxidation resistance.