The reasons for the special precautions just described have arisen from some recent work of Daniels and Loughlin who claim that commercial lard contains enough "A" vitamine to permit rats to grow, reproduce and rear young. The British authorities explain their results as not due to the presence of the "A" vitamine in the lard but to a reserve store in the bodies of the animals. They hold that animals may thus store the "A" vitamine but that apparently they have no storage powers for the "B" that are comparable to it. Osborne and Mendel repeated the experiments described by Daniels and Loughlin, using the purification methods just described, but failed to obtain similar results with either commercial lard or with the purified fraction. They question the validity of the British explanation but at the same time reiterate their belief that even commercial lard contains no "A" vitamine. Whatever the explanation of this particular phenomenon it is important that the basal diet be of purified materials and the methods just described supply the procedure necessary to attain that end. Before discussing the application of these diets to vitamine testing, attention is called to other basal diets developed by McCollum. This worker has paid especial attention to the deficiencies of the cereal grains and in particular to their salt deficiencies. In his basal diets, we find, as would be expected, special combinations particularly suited to the detection of vitamines in such cereals. McCollum has also devised a method of extracting substances to obtain their "B" vitamine and of depositing it on dextrin. For that reason he uses dextrin instead of starch for his carbohydrate and when he wishes to introduce the "B" vitamine it can be done by his method without having to recalculate the carbohydrate component. His method consists of first extracting the source with ether and discarding this extract. Pure ether will not remove the "B" vitamine. The residue is then reextracted several times with alcohol and the alcohol extracts combined. If now these alcohol extracts are evaporated down on a weighed quantity of dextrin the activated dextrin can be used not only to supply the carbohydrate of the ration but also to carry the "B" vitamine of a given source that is under investigation. McCollum's basal diets and salt mixtures are tabulated in the following chart: _McCollum's basal diets and salt mixtures_