In acid solution, tissues tend to acquire a net positive charge and bind acid dye; binding of basic dye is enhanced in basic solution where tissues tend to possess a net negative charge. A measure of the acid or basic strength of a given tissue structure is afforded by its ability to bind basic or acid dye over a range of pH.

The capacity of blood cell components to bind light green and methylene blue in the pH range 0.05 to 12.3, and their affinity for eosin after acetylation and treatment with nitrous acid, procedures used to block or destroy primary and secondary amines and guanidino groups, was studied. Observations were also made of blood smears stained with the Serra test for arginine.

Erythrocytes bound light green through pH 12.3 and methylene blue to pH 6.0. The ground substance of white blood cells and immature blood cells stained with light green through about pH 9 consistently, and often to pH 11: methylene blue was bound through approximately pH 4.0. Eosinophilic granules bound light green through pH 12.0, and did not bind methylene blue. Basophilic granules bound methylene blue in the pH range 8.0 to 1.0 but were dissolved in more acid and basic solutions: they bound no acid dye. Neutrophilic granules were stained with methylene blue through pH 5.3. Nuclei of immature blood cells and monocytes bound methylene blue to about pH 4.0: polymorphous nuclei and nuclei of lymphocytes and normoblasts were stained very faintly to pH 0.05. Many nuclei bound light green to about pH 12.0.

Nuclear and cytoplasmic acidophilia was reduced or destroyed by acetylation or treatment of smears with nitrous acid. Nuclei and cytoplasm of blood cells were stained with the Serra test. Eosinophilic granules were stained strongly.

The cytochemical significance of certain experimental findings have been discussed.

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