1. Sterile broth extracts of unwashed pneumococci actively destroy hemoglobin to methemoglobin and lower degradation products.

2. Sterile saline extracts of washed pneumococci do not by themselves form methemoglobin; extracts of this type may be completed or activated by the addition of certain complementary substances such as meat infusion and yeast extract.

3. The hemoglobin-destroying activity of pneumococcus extract is lost by exposure to 65°C. for 10 minutes.

4. The properties of an extract upon which these blood changes depend are related to other known oxidation-reduction functions of the same extract.

5. Oxyhemoglobin is converted to methemoglobin only by cell extracts in the reduced form; completely oxidized extracts are inactive in the presence of blood. The action of hydrogen peroxide and the influence of blood catalase on these reactions are discussed.

6. During the reaction between oxyhemoglobin and pneumococcus extract oxygen is consumed.

7. The mechanism of methemoglobin formation by pneumococcus is interpreted as an oxidation process in which deoxygenation of oxyhemoglobin and peroxide formation occur as intermediary reactions. The active agent of the oxidation of hemoglobin is considered to be a peroxide of bacterial origin.

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