The paper gives the results of a series of experiments made with a special technique for the purpose of testing whether or not so called antiblastic phenomena or bactericidal phenomena play any part in immunity to pneumococcus. The technique employed is that devised by the writer and consists in the isolation of single bacteria and their growth in hanging drops where growth can be continuously observed. From these experiments it may be concluded that: (1) Whole fresh blood, coagulated plasma, or serum of the immunized horse added directly to pneumococci, has considerable inhibiting action on the growth of pneumococci. The inhibiting effect of the fresh blood, coagulated plasma, or serum of a normal horse, however, is as marked as that of a horse highly immunized to pneumococci of the same type. This property, therefore, does not seem to be of importance in acquired immunity in the horse. No such property has been demonstrated in whole fresh rabbit blood. (2) It has been impossible to demonstrate that antiblastic phenomena play any part in natural immunity to pneumococcus, at least as far as the immunity of the pigeon is concerned. The whole fresh blood of the pigeon under the conditions employed exhibits no inhibiting action on the growth rate of pneumococci.

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