Eight healthy male volunteers supplemented their normal diet with 10-15 g/d of a fish oil supplement (MaxEPA) to provide 1.4-4.2 g/d of eicosapentaenoic acid (EPA; C20:5 n-3) for a period of 12 weeks. Blood samples were taken at weeks 0, 2 and 12 and the fatty acid compositions of the phospholipids of plasma, platelets, neutrophils, monocytes and T- and B-lymphocytes were determined. In all instances the level of EPA increased significantly (P < 0.05) by 2 weeks and remained so without a further increase for the ensuing 10 weeks. Beyond that, few consistent patterns in fatty acid composition were observed. Arachidonic acid (C20:4 n-6) fell significantly (P < 0.05) in plasma, platelets, neutrophils and T- and B-lymphocytes, but generally tended to do so only by week 12. Given the wide variability in the half-life of these cells (minutes for neutrophils, months for lymphocytes) it is evident that uptake of plasma EPA occurs by phospholipid exchange into preformed mature cells and does not require incorporation during cell genesis.