There were three principal phases of protein nutrition which Osborne and Mendel discussed in the light of their data. The importance of their deductions is so great and the defects in their method so difficult for the inexperienced to appreciate, that a somewhat full discussion is warranted of the manner in which "protein-free milk" vitiated a considerable part of a research of a most comprehensive type. Three phases of protein metabolism will be discussed here: The comparative values of the individual proteins in growth; the protein requirements of the animal body in maintenance as contrasted with growth; the possibility of the existence for the maintenance of the species of a synthetic capacity in the maternal organism, through the medium of the placenta and of the mammary gland, which cannot be affected by other body tissues for the maintenance of the life of the individual. These will be considered in the order enumerated.