That the subject of immunity offers a wide field for research, becomes the more apparent the further one investigates this complex question in its various aspects. In fact, so wide is the field presented to the reader and so numerous the channels into which it has been divided by the investigations of more recent years, that it has become a matter of no little difficulty to decide to which of the many aspects of this complex problem one should more particularly devote their attention. In the broadest and most general terms, the subject of immunity from the point of view of research might be said to present the following great problems: I. The QUESTION of TOXIN and ANTITOXIN FORMATION. II. The QUESTION of. BACTERIAL ACTION and the DEVELOPMENT of ANTIBACTERIAL PROPERTIES in the ANIMAL BODY. III. The QUESTION of PHAGOCYTOSIS. IV. The QUESTION of HAEMOLYSIS and the PRODUCTION of ANTI-HAEMOLYSINS and other allied LYSOGENIC ACTIONS. Of these problems, the first has probably so far met with the most satisfactory explanation and it might also be said, has furnished the starting point of most of the work which has already been done in other fields of immunity. In this connection, Ehrlich's theory has met with such wide-spread acceptance and would seem to explain the various phenomena so satisfactorily that all other theories have become dwarfed in consequence. With regard to the second problem, namely: - the question of Bacterial action and the production of Anti -Bacterial substances, it cannot be said that the same satisfactory position has been reached. We have here so many factors to deal with that the difficulties become proportionately great, and so the number of theories which have been advanced, to explain the various phenomena individually and collectively, have naturally assumed enormous proportions. In the main the two great theories which might be said to hold the field are, on the one hand, the Cellular theory, supported by letchinkoff and his fellow -workers and, on the other hand, the Humoral theory, supported by Ehrlich and his followers and, which is, in effect, an adaptation of Ehrlich's original antitoxin theory. The third great problem presented in the study of immunity is the question of Phagocytosis, more especially in its relation to the Phagocytosis of Bacteria. The fourth and last great problem, namely: - the question of Haemolysis and other Lysogenic actions cannot be lost sight of on account of the valuable parallel which it forms to Bactericidal actions in general, and much valuable information has already been discovered as a direct result of the study of this analogous process. Amongst the many other problems suggested by a study of immunity, one might perhaps mention, Natural Immunity, also Acquired Immunity, active and passive, their duration and the variations in this duration in the case of different Bacteria, its mode of production and the cause of its persistance in some cases and not in others and also the question of recovery from Bacterial Infection. Every one of these problems might be said to require further investigation as none of them would seem to have so far met with an entirely satisfactory explanation. Nor are these, by any means, the only problems requiring investigation for, on each of these are dependent innumerable side issues, all of which call for careful study and research. From all these problems the question of Phagocytosis has been selected for special study and to it attention has been entirely confined in this work, except in so far as a study of the other problems has helped to elucidate many of the phenomena observed in the case of the former. In regard to the question of Phagocytosis, attention has been more especially directed to a study of that particular constituent of the Blood, comparatively recently discovered by Sir A .E.Wright and designated by him opsonin. In concluding this brief introduction, I should like to express my indebtedness to Professor Greenfield for the privilege of carrying out this re- search in the Pathology Department of the University. I have also to acknowledge a grant from the Earl of Moray fund in aid of this work.