Summary: The above experiments therefore show that with regard to: The highly virulent streptococcus. The immunised animal can completely clear the peripheral circulation and prevent the occurrence of secondary waves of bacteriaemia, in the absence of (1) agglutinin, (2) any germicidal power of the whole blood, and (3) any protective antibody capable of conferring passive immunity.B. anthracis. The same results as a.Pneumococcus type III. The immunised animal can clear the circulation and prevent the occurrence of secondary waves of bacteriaemia even though (1) the blood cannot confer passive protection on mice, (2) the blood is not germicidal (except occasionally, experiment 25), (3) the blood does not have any appreciable power of conferring passive protection on rabbits (when the results are compared with the effect of the inoculation of normal sera, etc., into normal rabbits), and (4) the serum contains no agglutinin nor precipitin against the organism.Animals immunised a long time previously with pneumococcus type I, where all traces of agglutinin had disappeared and where there was no germicidal or protective power to cause passive protection in mice or rabbits, completely cleared the circulation of the actively growing virulent organism.With the B. Friedländer, the same was noted in animals immunised long previously and which had completely lost their agglutinating antibody. All these experiments taken together irresistably lead to the conclusion that the chief factor in immunity is the state of the tissues with regard to the infecting bacteria in question.