Thyroidectomized foxhounds were fed: (a) a control diet, (b) the control diet supplemented with 15% beef tallow or cottonseed oil, with or without added cholesterol, or (c) the control diet supplemented with beef tallow (15%), safflower oil (1.5%) and cholesterol. After 23 weeks on the diet, the content of the individual phospholipids of the erythrocytes was not altered appreciably. However, supplementing either the saturated (beef tallow) or polyunsaturated (cottonseed oil) diets with cholesterol produced similar changes in the fatty acid compositions of the phospholipid classes. Most consistently, there were increases in the percentages on 18:2 omega 6 and 20:3 omega 6 and decreases in the percentages of 20:4 omega 6 and 22:4 omega 6. Cholesterol supplementation of the diets similarly affected the fatty acid composition of phosphatidylcholine isolated from the platelet-free plasma of these animals.