1. The effect of immune serum upon the electrical charge on bacteria has been studied.

2. Immune agglutinating serum possesses a specific charge-reducing effect which is quantitatively related to the agglutination titer of the serum. This effect is lost when the serum loses its agglutinating power, that is after adsorption of agglutinin by homologous bacteria; adsorption by heterologous organisms does not affect this property.

3. A highly protective, non-agglutinating serum did not show this specific charge-reducing effect.

4. It is suggested that the reaction may have practical diagnostic application.

5. The findings of Northrop and De Kruif, that unsensitized and sensitized bacteria agglutinate only when the charge on the bacteria has been reduced to a critical potential zone lying between +15 and –15 millivolts, have been confirmed in the case of all electrolytes tested except Na2HPO4.

6. When Na2HPO4or phosphate buffer solutions are used as electrolytes, specific bacterial agglutination occurs at negative charges well above –15 millivolts, and with serum in high dilution, specific agglutination takes place without any observable reduction of charge.

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