The present paper reports the influence on plasma lipids of isoenergetic diets containing 30 YO of energy as fat, with a polyunsaturated: saturated fat ratio of 4.00 or 0.25, consumed for 8 weeks by twenty-five young normolipidaemic males. Approximately 70 % of the fat energy was provided by the test fats: soyabean fat and coconut fat. During the soya-bean-fat-eating period the total plasma cholesterol level fell significantly compared with baseline values (P < 0.001) and during the coconut-fat-eating phase total plasma cholesterol level increased significantly compared with the soya-bean-eating period (P < 0001).On the soya-bean-fat diet, high-density-lipoprotein (HDL)-cholesterol decreased by 15 YO (range 6-35 YO) and plasma triacylglycerols decreased by 25 YO (range 13-37 YO). Results of the present study show that even when the proportion of total fat in the diet is low, a high intake of linoleic acid lowers both total plasma cholesterol and HDL-cholesterol, while a high intake of saturated fat elevates both these lipid fractions. Application of regression formulas to the present findings indicates that short-chain saturated fatty acids have a neutral effect on serum cholesterol.