The results of the simple experiments that I have summarized emphasize the value of the precipitin reaction in studying certain constituents of the body for various purposes. The reaction may be of service to physiological, chemical and immunological research as well as for diagnosis. Of the substances that have been considered in the foregoing, the serum proteins, hemoglobin, and the specific precipitinogen in semen appear to be distinctive or specific for the species, and consequently presumably different chemically in each species, while the lens proteins, fibrinogen and thyroglobulin appear to be more or less closely related in many distinct species. Of this latter group, the lens and casein2 have been the standard illustrations to which we now can add fibrinogen and thyroglobulin. That fibrinogen should disregard so completely the species limitations of the immune reactions of serum and its constituents as well as of hemoglobin is both surprising and interesting. In fibrinogen the blood reveals a much wider antigenic relationship than is ordinarily assumed to be the case. It is of special interest that in connection with the fundamental problems of immune specificness and the production of antibodies these nonspecies-specific antigens can evoke the formation of corresponding antibodies in the homologous species, at least under certain conditions. In view of this circumstance may it not be well to subject to renewed experimental scrutiny the old assumption that species-specific antigens are incapable of provoking any antibody response in their own species?