1. A study has been made of natural agglutination as exemplified by the reactions of the serum of nine animal species with a variety of bacteria.2. End-titres are recorded, and the fact is noted that sera of different animal species show an order of agglutinating activity which is almost constant for all organisms used. Ox, pig and horse sera give consist - :ently strong reactions, while specimens from rabbit, guinea -pig and rat react weakly or not at all. Sheep, human and cat sera occupy an intermediate position. Variations are noted, however, with different individual specimens of serum from the same species.3. Organisms of the series tested can also be grouped in order according to their apparent susceptibility to agglutination by normal sera.4. The serum of young animals is found to be deficient in the agglutinating principle.5. The agglutinating effect shows a thermolability intermediate between that of complement and the immune agglutinins. Complete inactivation occurs as a rule after exposure to 60 °C. - 65 °C. for half -an -hour. For certain strains the serum principle is inactivated at much lower temperatures.6. Lability curves show marked irregularity. In certain cases a zone of relative inactivation is produced at a temperature of 55 °C.7. The natural agglutinating substance is found to be present in greater degree in the carbonic acid in- :soluble fraction of serum than in the carbonic acid soluble fraction, In this respect it differs from the immune agglutinins, which are chiefly located in the carbonic acid soluble moiety.8. The agglutinating principle for each organism can be absorbed completely by the homologous strain, when a variable lowering of the end -titre for other unrelated organisms results. A similar lowering of activity for these organisms may be produced by treating the serum with non -specific physical absorbents. Charcoal and Kieselguhr were used to demonstrate this.9. By the technique of double absorption it can be shown that agglutination depends on non-specific and specific factors and it is concluded that normal serum agglutinates bacteria in virtue of a twofold mechanism: (a) A non-specific effect reacting in varying degree with all organisms and removable by treatment with a finely divided absorbent. (b) A series of specific effects reacting as true "natural antibodies ". These specific antibody -like principles exist for a wide variety of organisms. Absorption of any one organism removes the homologous effect leaving the remainder quantitatively unimpaired.10. Normal serum from various mammalian animals contains agglutinins which react with the "H" and "0" antigenic constituents of motile bacteria. Flagellar suspensions have been used to demonstrate H- agglutinins.11. Agglutinin- absorption experiments show that the specificity of natural agglutinins depends chiefly on the "H" type. The "0" type agglutinins appear to possess affinities for antigenic constituents which are more widely shared by different organisms.12. It was not found possible to demonstrate the antigenic relationship among members of the Salmonella and B. proteus X groups so precisely with normal sera as with immune sera.13. The thermolability of the 0- agglutinins was found to be greater than that of the "H" type in the normal serum of a number of animal species. Both showed greater lability than the corresponding immune agglutinins.14. "Rough" and "smooth" variants of the same bacterial strain showed antigenic differences in their reactions with normal sera.15. The origin of natural agglutinins is discussed in its relation to other natural immunity reactions.16. The suggestion is made that true natural antibodies must be contrasted with specific immune anti- bodies in the serum of apparently normal animals. .After reviewing the available evidence, the conclusion is reached that the former cannot be regarded as a response to a specific stimulus. The most satisfactory explanation of their presence is one which suggests that they arise spontaneously in the course of the serological development of the animal
