The interactions that occur between bone and metal are exceedingly complex. Almost without exception, past experiments have been limited to observation of the changes that occur in bone when the metallic foreign body is inserted in it. The problem cannot be solved from this point of view alone. The question does not involve merely the reaction of bone to metal but is concerned as much with the changes that occur in the metal when it is buried in bone. A basic consideration is that the exact chemical composition of the metal tested must be known. Although this seems obvious, the omission of data on the composition of the metal from previous studies probably accounts for variations in the results. Only a few metals are relatively pure, the majority containing other substances in varying amounts; and alloys, or metal mixtures, which seem closely related may be chemically dissimilar. The inclusion of