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This paper presents data on the plasma cholesterol ester fatty acid composition (1) in patients studied in the metabolic ward during quantitatively constant intakes of either purified ethyl oleate or linoleate, and (2) in 35 normal males and 6 normal females whose diets were "general" a t the time blood was obtained for analysis. MethodsExtraction procedure. Fasting plasma lipids are extracted with 25 volumes of solution containing equal parts of ethanol and acetone. The plasma is added slowly to the solvent with shaking; this is brought to a boil and filtered rapidly, using fat-free filter paper. Aliquots of this extract are used for the column separation of the cholesterol esters.Preparation of solution in petroleum ether. Two 50-ml. aliquots of the plasma filtrate, each representing 2 ml. of plasma, are evaporated to dryness under partial vacuum at approximately 50" C. Five ml. of petroleum ether is added to the residue to dissolve the fat. This is added to the column. The tube is washed 3 times with 2-ml. portions of petroleum ether and these portions are added to the column. After the absorption of the petroleum ether by the Celite-silicic acid, 100 ml. of petroleum ether is added for complete elution of the cholesterol esters. The rate is adjusted by application of slight positive pressure to attain a flow rate of approximately 1 ml./min.Preparation of the column. The column consists of a 25-ml. burette with a self-lubricating Teflon plug assemblyt and a 125-ml. flask fused to the top of the burette as a reservoir. A glass wool plug is introduced and a mixture of equal parts by weight of Celite and silicic acid is added with tamping to a height of 10 cm.Many investigators who use silicic acid for the separation of the various fat components advise the use of certain mesh sizes and activation by heat' or by dehydrating solvents? Such activation will increase the efficiency considerably if one is interested in obtaining the maximum amount of lipid adsorption per gram of silicic acid. However, dehydration of silicic acid below a certain point will eliminate its adsorptive power almost completely. The degree of activation affects not only the amount of fat adsorbed per gram, but also the elution of certain fats. When 100 ml. of petroleum ether is added to the col-* The work reported in this paper was supported in part by grants from The Armour Laboratories, Paterson, N. J.; Carnation Company, Los Angeles, Calif.; Schering Corporation, Bloomfield, N. J.; Alameda County Heart Association, Oakland, Calif.; and the National Institutes of Health, Public Health Service, Bethesda, Md. t "Ultramax," Fischer and Porter Company, Hatboro, Pa. 633