The studies of Leishman, of Wright and Douglas, of Neufeld and Rimpau, of McFarland and Engle, and of others, have opened up a new field of investigation for the elucidation of the problem of the mechanism of immunity. The experiments that have been reported thus far have dealt almost entirely with the opsonic powers of the blood serum against staphylococci, streptococci and pneumococci, and the observations have been made exclusively on the phagocytic powers of the polymorphonuclear leucocytes, or the microphages of Metchnikoff. It seemed desirable to ascertain whether the opsonic power of the serum was efficacious against other classes of micro-organisms, and also whether the macrophages of Metchnikoff participated in the phagocytosis. Metchnikoff himself states that in the resorption of animal cells in general it is especially the macrophages which intervene, but in natural immunity against micro-organisms positive chemiotaxis is exhibited by the microphages more than the macrophages. He