Many theories have been advanced in explaining the cause of the clumping and precipitation of bacteria by their immune serum. It is generally known that the presence of salt solution is also necessary. In many text books of bacteriology the statement is made that acids and alkalis inhibit the agglutination reaction, but amounts necessary to do this are not given. It has been observed in the present work that alkalis have a stronger inhibiting action than acids. Ammonium hydroxide has very little effect as compared to sodium or potassium hydroxide. The weak acids however have as strong an inhibiting action as the stronger acids. Lactic and butyric acids inhibit in high dilutions
