Savtchenko,1 Levaditi,2 Gruber3 all observed that injection of serum of rabbits immunized with guinea-pig blood into the peritoneal cavity of guinea-pigs is followed by a marked erythrophagocytosis not only in the abdomen, but also in the blood-making organs, especially the spleen, and even in the circulating blood (Levaditi). In this phagocytosis take part microcytes as well as macrocytes. Ruziczka noted the occurrence of phagocytosis in vitro of red corpuscles in the presence of the corresponding immune serum.4 This phagocytosis was ascribed by Savtchenko to the action upon either the phagocytes or the erythrocytes of the amboceptor or substance sensibilisatrice in the immune serum, and this explanation is not contested by either Levaditi or Gruber. A year ago Neufeld and Topfer5 showed that the blood of rabbits immunized with goat blood contains a substance that by action on goat corpuscles after absorption by them renders them subject to phagocytosis by guineapig leucocytes in vitro. This substance, which they designated as hemotropic, had no direct action on the leucocytes. Elsewhere6 I have pointed out that such substances should be called opsonins (hemopsonins) because of their analogy to the bacteriopsonins of Wright and Douglas who were the first to show clearly the opsonic function of the serum in phagocytosis of bacteria. Barratt7 found that doves immunized with hen blood give a serum that is strongly opsonic for hen corpuscles and demonstrated that in this, as well as other cases, the immune serum acts upon the erythrocytes and not directly upon the phagocytes. Neufeld and Topfer, as well as Barratt conclude that the hemotropic substances, or hemopsonins, are distinct