1. Sterile filtrates of aerobic cultures of pneumococcus which contain hydrogen peroxide are capable of converting catalase-free solutions of crystalline oxyhemoglobin into methemoglobin. This action is dependent upon the hydrogen peroxide in the filtrates and occurs only in the absence of blood catalase.
2. In catalase-containing solutions of hemoglobin from laked corpuscles, the actual methemoglobin-forming system of Pneumococcus involves a labile constituent of the bacterial cell. This intracellular substance is itself susceptible to oxidizing agents and may be rendered inactive, if exposed to peroxide or similar substances previous to its introduction into oxyhemoglobin solutions. The activity of this function in the case of sterile filtrates of pneumococcus cultures depends, therefore, upon the liberation of cell constituents into the medium and upon the protection of these cellular substances from the oxidizing agents which are formed when pneumococcus cultures are freely exposed to air. When these cultural conditions are fulfilled sterile culture filtrates of Pneumococcus convert oxyhemoglobin into methemoglobin independent of the presence of blood catalase.