1. Actively growing pneumococci produce acid at such a rate that change in the pH of the medium parallels change in rate of growth. 2. The death of the pneumococci is not followed by change in the reaction. 3. Acidification during growth in beef infusion media proceeds until a pH of about 7 is reached. At this point growth stops. The increase in hydrogen ion concentration is not the only origin of the injury to the cell that causes lag in a subsequent culture. 4. The increase in hydrogen ion concentration of the medium is not the sole cause of cessation of growth.