Although Bacterium pneumosintes is a stable organism which agglutinates only in low dilutions of the serum of actively immunized rabbits, the serological reactions of this microorganism have always been found to be consistent and specific. The results of the agglutination and precipitation reactions described in this paper indicate that the serum of normal persons does not contain demonstrable agglutinins or precipitins for Bacterium pneumosintes. By contrast, agglutinins have been demonstrated in the serum of seventeen persons among nineteen who were examined from 10 days to 5 months after recovery from epidemic influenza. The serum of ten persons who had influenza, followed in three instances by pneumonia, 2½ to 3½ years before, proved negative. In one instance the appearance of specific agglutinins against Bacterium pneumosintes was found to be coincident with an attack of uncomplicated influenza. In twelve of fifteen instances in which agglutinins were found, precipitins against Bacterium pneumosintes were demonstrated also. It is noteworthy that these antibodies may persist in the blood at least 5 months after recovery from the disease.

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