It is now quite generally recognized that certain substances when dissolved in a solution in which leukocytes are suspended, markedly affect the ability of these cells to phagocyte. The action of some of these compounds on phagocytosis is accelerative; thus, for example, the current notion of opsonic effect has for its foundation the idea that the bacteria present in conjunction with the leukocytes become sensitized or in some way altered so that the white cells may more readily engulf them. Even though we grant the correctness of this claim of Wright and Douglas,1 that the opsonins act specifically on the bacteria and not on the leukocytes, the exact explanation of the mechanism of the process has never been forthcoming. There are, however, many facts which indicate that the phagocytic power of the leukocytes on inert suspensions, such as charcoal, dead bacteria, etc., can be markedly influenced in various