1. Normal “whole” blood and serum-leucocyte mixtures may exert a bactericidal effect when the serum (or plasma) is inactive in this respect, and may possess quantitatively greater bactericidal properties than serum (or plasma), when the latter is active per se, but such effects are relatively weak.
2. Leucocyte suspensions may exhibit an initial bactericidal action on certain bacteria, but this is usually weak and is followed by a pronounced growth-promoting influence on the surviving organisms.
3. Under certain conditions normal “whole” blood and serum-leucocyte mixtures are inferior to serum (or plasma) in bactericidal power. This is observed when the serum (or plasma) is strongly active per se and is due not merely to an inhibition of the bactericidal action of the leucocytes, but also to a lessened effect of the serum-bactericidins.
4. The comparative bactericidal power of normal “whole” blood or serumleucocyte mixtures on the one hand and serum or plasma on the other depends on the activity of the serum; when this is relatively weak, blood shows an enhanced action; when strong, blood is less active.
5. The above findings apply to a wide variety of bacteria irrespectively of biological species.
6. Bacterial extracts possess marked inhibitory properties on the bactericidal action of serum (or plasma), but not to any extent on the bactericidal properties of the leucocytes as evidenced by the approximately equal influence of such extracts on whole blood and serum respectively.
7. Dead bacteria in certain numbers produce inhibition of the bactericidal action of serum (or plasma), but exert a greater inhibitory influence on the reaction of “whole” blood.
8. It is suggested tentatively that when the serum is strongly bactericidal the killed organisms on being phagocyted along with living bacteria interfere with the intracellular destruction of the latter which at the same time are protected from the serum-bactericidins.
9. The paper also includes certain data regarding the time of maximum bactericidal action by serum towards various organisms and the occurrence of growth-promoting action following an initial bactericidal effect.