A Study by the Single Cell Method of the Influence of Homologous Antipneumococcic Serum on the Growth Rate of Pneumococcus

1. Under the conditions stated pneumococci grow as readily in the serum of horses highly immunized to the homologous organism as they do in normal horse serum, and the rate of growth is not appreciably diminished. 2. This failure of immune serum to affect the growth rate is not altered when fresh rabbit blood, fresh human blood, or rabbit blister fluid is added in order to supply any hypothetical complement which might be lacking. 3. We have not been able to show that when immune horse serum is injected intravenously into rabbits or intraperitoneally into mice, it acquires the property of killing pneumococci or inhibiting their growth. 4. Experimental evidence has been obtained indicating that in the peritoneal cavity of the passively immunized mouse the growth of extracellular pneumococci continues at apparently the normal rate, until the bacteria are engulfed by phagocytes. 5. The immunizing and protective power of antipneumococcic serum probably depends, in part at least, on properties which are not at present known. It has not been possible in the present study to demonstrate that one of these properties consists in delaying the growth of pneumococci.