Background: Membrane fluidity is an important aspect of cellular physiology which may be manipulated by diet. Methods: We studied the effect of dietary fish oil on the membrane composition of erythrocytes and cheek cells, and on membrane fluidity of erythrocytes as assessed by fluorescence recovery after photobleaching (FRAP). Healthy volunteers received a daily supplement of fish oil (930 mg EPA, 630 mg DHA) for 42 days. Results: The intervention reduced the ratio of n-6 to total fatty acid in the phospholipid fraction of erythroctye membranes but the n-3 fraction remained stable and the ratio of cholesterol to phospholipid increased. The level of EPA and DHA in cheek cells increased significantly during the intervention period. The mean diffusion coefficient of the fluorescent probe in erythrocyte membranes increased from 7.2 ± 0.7 x 10-9 cm2/s at the start to 9.8 ± 0.5 x 10-9 cm2/s after 21 days. Membrane fluidity remained higher than the initial value 42 days after withdrawal of the supplement. Fish oil also reduced platelet aggregation in response to ADP but there was no effect on plasma lipid profiles. Conclusion: We conclude that n-3 fatty acids influence erythrocyte membrane composition at relatively low levels of supplementation by a mechanism which does not necessarily involve an increase in the level of EPA or DHA present in the membrane.