When this paper appeared McCollum had already accumulated experimental data which led him to believe that there were very great differences in the nutritional value of fats from several sources. With diets composed of protein, starch, milk sugar, salts, and fats, it had been found that prolonged growth was secured when butter fat was added. This had been dissolved in ether and passed through filter paper in order to make certain that no traces of any ingredients of the milk except the fats and substances having the same solubilities were present. Egg yolk fats behaved like the purified butter fat, whereas little growth and early failure resulted when olive oil or lard formed the only fats in the diet. Owing to an epidemic which destroyed the rat colony a year elapsed before a fairly satisfactory demonstration of this fact could be secured, the records of which seemed safe for publication. In June, 1913, McCollum and Davis (8) recorded their findings that carefully purified butter fat, all of which was soluble in fat solvents, and likewise egg yolk fat, contained something which greatly promoted growth, and that lard and olive oil did not possess this property.