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Some time ago we described two diets, Nos. 2638 and 2677, which we used with a view to the development of a biological test which would show the calcium-depositing power of any given substance.' The test was carried out as follows: The faulty diet was first fed to a group of young rats for the purpose of making the epiphyseal cartilage free from calcium, and producing a rachitic metaphysis. After a sufficiently long period had elapsed, the test substance was added to the diet of those animals which were to serve as test subjects, while the faulty diet without the test substance was continued in the case of the control rats. Substances, which when added to the faulty diets enabled the organism to deposit lime salts, caused the reappearance of the provisional zone of calcification in the bones. This biological test we called the "line test," because the new provisional zone of calcification appeared as a line of calcium salts extending transversely across the bone with a limeless cartilage on one side of it and a limeless metaphysis on the other.The success of this test depends on the use of a diet which uniformly causes the epiphyseal cartilage and the metaphysis to be free from calcium salts. I t is not sufficient that a diet should merely produce rickets. The rickets which it produces must be of so severe a type that no vestige of calcium remains in the cartilage, and a wide metaphysis is formed. Moreover, the diet must be so constituted that the animals restricted to it will grow and maintain a fair state of general health and nutrition. The diets which we earlier described were not satisfactory, since they did not invariably produce typical rickets. Now and again an animal which was restricted to one of them would be found whose