1. By a suitable bactericidal technique, it can be demonstrated that the virulent S influenza bacillus is completely resistant to the bactericidal action of diluted normal unheated serum. In contrast, the R organism is easily killed when subjected to the action of diluted normal serum. Although this is not a true virulence test, it promises to be a useful substitute when a susceptible animal is not available.
2. The S culture filtrate contains a substance with a strong antibactericidal effect, but the R culture filtrate does not contain this substance.
3. It would appear probable that this antibactericidal substance is identical with, or closely related to the precipitinogen (or soluble substance) which is present only in the culture fluid of the S influenza bacillus. In view, however, of differences in heat stability of the two substances, this question must remain in doubt until a supply of purified precipitinogen is available.
4. Quantitative agglutination, precipitation and bactericidal comparisons between (a) R-absorbed anti-S serum and unabsorbed anti-S serum and (b) anti-S serum and anti-R serum indicate that the agglutinogen is a separate antigen, and the agglutinin a separate antibody, taking no part in the bactericidal action of the antiserum. They also indicate that the precipitin, which is present only in the anti-S serum, is identical with the bactericidal antibody.
5. The relation of the precipitinogen (or soluble substance) to virulence and of the precipitin to resistance is discussed.