2. Cultures of pneumococci in the fresh whole blood of immune animals, as compared with cultures in the blood of susceptible animals, show a greatly prolonged latent period, and, in a general way, the relative lengths of the latent periods of the cultures correspond to the relative resistances of the animals to infection by these organisms.

3. The blood of animals artificially immunized, both actively and passively, retards the growth of pneumococci in the same manner as the blood of naturally immune animals.

4. Microscopic examination of cultures of pneumococci in immune blood reveals chain formation, growth in clumps, and phagocytosis of the organisms by the polynuclear cells. It also shows that growth occurs first in the free serum, the clot being invaded later.

5. The retardation of multiplication depends on two factors, opsonization of the pneumococci by the immune serum and phagocytosis of the organisms by the polynuclear cells; growth readily occurs when either agent is absent.

6. Pneumococci multiply in defibrinated immune blood because few phagocytes are present after defibrination.

7. Pneumococci grow in the most potent immune blood after mechanical destruction of the white cells.

8. It has not been shown that immune blood does not kill a certain number of the pneumococci with which it is inoculated, but the tentative conclusion has been arrived at that no killing occurs since none of the tests became sterile during the course of our experiments.