Since the nitrogen contained in "protein-free milk" can act as a source of certain amino-acids, it is obvious that the true dietary value of any series of proteins which it is desired to compare cannot be secured, when they are studied by the method involving the concomitant feeding of this material. The scope of the studies of Osborne and Mendel, and the importance of the deductions which have been drawn from them are so great that a detailed exposition of the action of the nitrogenous constituents of "protein-free milk" in vitiating their results will be presented. This is best brought out by their studies of the nutritive value of gliadin, one of the principal proteins of the wheat kernel. These are of fundamental importance because their interpretation involves the question, not only of the biological value of "protein-free milk" as a source of amino-acids, and its bearing on the recorded nutritive significance of numerous proteins, but likewise on the problem whether the processes of maintenance without growth, or the repair of tissue waste, is of a lower order than those of growth requiring fewer amino-acids than the latter.