Hemoglobin is oxidized to methemoglobin by products formed during the oxidation of a number of different substances. With some of these substances, both oxygen consumption and hemoglobin oxidation are accelerated by thermolabile constituents of pneumococci; with others of the easily oxidized substances, neither reaction seems to be influenced by the presence of the bacterial substances. The same substances which in the presence of air induce the oxidation of hemoglobin bring about the reverse reaction if molecular oxygen is excluded.