The effect of the administration of the organic factor which has several times been mentioned as playing a role in the etiology of rickets, is very easy to demonstrate by properly planned experiments. If one places young rats on a diet which is too rich in calcium and too poor in phosphorus and in fat-soluble A, they develop a condition in their bones which is characterized by the absence of calcium salts in the zone of primary calcification. The calcium is available in abundance and a certain amount of phosphorus is present in the food, but owing to the unfavorable ratio and the absence of the organic substance in question deposition of calcium salts is not possible. When young rats are prepared by such a diet so as to insure that there will be a calcium-free zone in their bones, they respond with calcium deposition at once when the deficient organic substance is exhibited in the diet. Two per cent of cod liver oil administered to such rats leads in the course of five to ten days to the appearance in sections cut longitudinally through the bones, of a fine line of calcium phosphate, which represents the beginning of healing of the lesion. Butter fat, even to the concentration of 50 per cent of the food mixture, fails under the condition of this test, to stimulate calcium phosphate deposition as effectively as does 2 per cent of cod liver oil. With this technic it is now possible in a simple and easy manner to test the anti-rachitic property of any food substance with respect to this organic factor. By changing the composition of the diet on which the animals are prepared, so that the calcium-free condition is determined by some other factor in the diet, e.g., calcium or phosphorus, it is possible to test the anti-rachitic effect of each of the dietary factors which are concerned with the etiology of rickets. This test is by far the most satisfactory one yet devised for the detection or approximate estimation of any of the vitamins.