The effect of dietary cod-liver oil on factors which characterize membrane lipid fluidity was studied. To six volunteers a daily supplement of 15 ml of cod-liver oil, providing 3 g of omega 3 fatty acids, was given for 2 weeks. Changes induced by the supplement in the fatty acid patterns of the individual erythrocyte phospholipid classes did not occur at the same rate or in the same degree. A rapid incorporation of omega 3 fatty acids in plasma lipids and in erythrocyte phosphatidyl choline, at the expense of linoleic acid, was seen, resulting in increased total unsaturation. A slower and quantitatively smaller incorporation of omega 3 fatty acids in erythrocyte phosphatidyl ethanolamine and phosphatidyl serine was seen. No change in the fatty acid pattern of sphingomyelin was seen. Withdrawal of the supplement for 2 weeks did partly reverse the cod-liver oil induced changes in erythrocyte phosphatidyl choline, while the changes in erythrocyte phosphatidyl ethanolamine and phosphatidyl serine lasted. Neither a change in distribution of erythrocyte phospholipid classes, nor in the erythrocyte cholesterol/phospholipid ratio was found. The observed changes in lipid composition are indicative of an increased lipid fluidity.