Of fundamental importance in immunity is the question whether only proteins are antigenic. Wells1 says, "as yet, it has not been finally established that any colloids other than proteins can act as antigens." Assuming then, that only proteins possess antigenic properties, it becomes of interest to inquire what there is in the nature or constitution of proteins to which this is due. Two characteristics of proteins have received the greatest attention in this connection; their colloid nature and their huge molecular weight. Attempts to explain the antigenic properties of proteins on the basis of their colloid nature have been largely speculative and consist of the application of the principles of colloid chemistry to theories of immune reactions. The efforts to determine the influence of the size of the molecule on the antigenic properties of proteins have led to numerous attempts to produce antibodies for the products of protein hydrolysis and digestion. Our understanding of the chemistry of immune reactions is limited by our knowledge of the chemistry of proteins. Each new insight into the nature and constitution of proteins is followed by a readjustment of our theories of immuno-chemistry. Underbill and Hendrix2 place the discovery of the phenomenon of anaphylaxis among the more important recent advances in the physiology of proteins. It has also furnished considerable impetus to the study of protein chemistry. For a satisfactory explanation of the anaphylaxis reaction, jt became necessary to determine by what changes proteins were rendered toxic, since it was well known that native proteins were of themselves nontoxic. This led to the theories of Vaughan, Friedmann and Friedberger that anaphylaxis is due to parenteral digestion of proteins, and that the symptoms are due to intoxication with the resulting products. To obtain evidence in support of these theories the products of protein