INTRODUCTION. Phagocytosis in its relations to healing and immunity has been discussed most actively from various points of view, and it is to some of the results of the most recent investigations in this field that I not without hesitation ask your attention. The demonstration by Wright and Douglas1of the presence in blood and other fluids of certain substances called by them opsonins,2which render various bacteria susceptible to the phagocytic action of leucocytes, has given a fresh interest to the study of phagocytosis. At present we may accept as an established fact that phagocytosis of many bacterial and other cells by the leucocytes, in the first instance, is dependent on special substances, normal and immune, which become attached to the cells in question and in some manner so change them that they are taken up readily by polynuclear leucocytesin vitro. Leucocytes freed from serum do