The Pneumococcus is capable of oxidizing hemoglobin to methemoglobin and of reducing the latter to hemoglobin again. Both the living organism and sterile extracts will accomplish the changes; and like systems seem to be involved in them. The type of reaction induced is determined by the presence or absence of molecular oxygen.

The oxidation and reduction of the blood pigments are reversible processes. The equilibrium between hemoglobin and methemoglobin in a mixture of blood and pneumococcus cellular substances may be shifted in either direction at will by regulation of the oxygen tension.

In the present paper quantitative evidence is presented of the reduction of methemoglobin to hemoglobin by sterile animal tissues. The possible relations of this activity to the disappearance of methemoglobin from the blood stream of the living animal are discussed.

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