In men in calorie equilibrium, changes in dietary fat produce responses in the serum cholesterol level that, on the average, are predictable from the percentages of total calories provided by saturated (S) and polyunsaturated (P) fatty acid glycerides in the diets concerned. S and P have opposing actions and, in general, Δ Cholesterol (mg./100 ml.) = 2.7ΔS − 1.3ΔP, where Δ refers to the difference between 2 diets. Increasing the number of double bonds beyond 2 in polyunsaturated fatty acids does not result in proportionate increases in serum cholesterol-lowering effect. The mono-enes oleic and erucic acid have little or no effect on the serum cholesterol level when they are exchanged in the diet for equal calories of simple carbohydrate. Changes of fats in the diet produce serum cholesterol responses that are also correlated with the iodine values, or the square-roots of those values, of the fats concerned when the iodine value happens to be highly correlated with 2.7S - 1.3P. When changes in diet fats involve substantial differences in amounts of mono-enes, or of fatty acids containing more than 2 double bonds, the serum cholesterol response has a low or negligible correlation with the iodine value, or its square-root, of the fats.