McArthur and Luckett (11) misjudged as did Stepp in his earlier experiments, the peculiar nutritive value of butter fat as a source of an unidentified dietary essential. Stepp (12) added butter fat to a diet free from substances soluble in ether and alcohol, and observed that the mice in his experiments died just as early as they did on the lipin-free diet, whereas the same ration was made adequate by the addition of an alcoholic extract of egg yolk or calf brain. McArthur and Luckett did likewise. These investigators, like Stepp, drew the conclusion that butter does not contain an indispensable substance for the nutrition of the mouse. The explanation of their results was finally brought out by the studies of McCollum and Davis (5). When butter fat or an ether extract of egg yolk was added to a diet of purified food-stuffs, only one of the unidentified dietary essentials (fat-soluble A) was introduced. When an alcoholic extract of egg yolk was added, both fat-soluble A and water-soluble B were contained in the added material. Since the addition of either the factors A or B alone does not make a diet of purified protein, carbohydrate, fat and mineral salts complete, the animals fed such diets with but one addition died, and there was no evidence in the experiment to show that the butter fat had any special properties. It was difficult to discover that in the one procedure one essential substance, and in the other two, were being added, and that the presence of the two was the determining factor in the outcome of the experiment.