Using the method described in an earlier publication for testing the growth-inhibitory and bactericidal activity of serum-leucocyte mixtures for the pneumococcus, a study has been made of the action of antipneumococcus serum added to the serum and leucocytes of the rabbit, which alone lack the power to inhibit pneumococcus growth. It was found that the additiori of relatively small quantities of immune serum to a mixture of rabbit serum and washed rabbit leucocytes conferred on this mixture marked growth-inhibitory and pneumococcidal powers. The effect of the immune serum was found to be quantitative in nature. Beyond a certain point progressive dilution of the serum resulted in a corresponding decrease in the degree of growth inhibition, but this property of the immune serum did not disappear until a high dilution was reached. The leucocytes were also found to exert their effect quantitatively. Within certain limits the number of pneurnococci killed was dependent on the quantity of them present. Tests comparing the action of immune serum in the rabbit serum-leucocyte tubes with their protective action in mice showed a close parallelism between these two effects; that is to say, an immune serum showing marked protective properties for mice was likewise effective in high dilution in the in vitro test, and vice versa.

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