The effect of a daily supplement of 75-100 g fatty fish, providing 3 g of omega 3 fatty acids, on the lipid composition and deformability of erythrocytes was studied in six healthy male subjects. After 4 weeks of the fish supplementation the mean degree of unsaturation, expressed as double bond index, of erythrocyte phosphatidyl choline (PC) and phosphatidyl ethanolamine (PE) had increased significantly by 10% and 4%, respectively. The mean cholesterol/phospholipid ratio (C/P ratio) in the membranes had increased by 28% (p less than 0.001). No change was seen in the distribution of the four major phospholipid classes. After withdrawal of the supplement for 4 weeks the changes in PC were reversed, but the changes in PE persisted and the mean C/P ratio still was higher than basal by 16% (p less than 0.01). The effect on the C/P ratio by the fatty fish is in contrast with the previously observed lack in effect of cod-liver oil. The viscosity of erythrocyte suspensions with a packed cell volume fraction of 0.80 and measured at shear rates less than or equal to 8.11 s-1 was decreased after the fish. We suggest that the fall in the viscosity of erythrocyte suspensions, reflecting increased cell deformability, is probably due to the change in the fatty acid composition of erythrocyte PC.