“Soft” fats were found to depress significantly serum cholesterol levels in diets of high energy to protein (E/P) ratio but not in diets of low energy to protein ratios. A low (E/P) ratio depressed serum cholesterol values, regardless of the type of fat in the diet. The E/P ratio may represent the unknown factor in the equations of workers who have tried to relate the effect of dietary fats to serum cholesterol levels.