Recent studies by Martin Mooney and Winslow, and Clifton,, on the metabolic activities of bacteria indicate that the rate of metabolic activity per cell varies widely at various phases of the growth-cycle. The maximal oxygen-consumption, carbon dioxide-production or ferricyanide-reduction per cell per unit-time was noted near the end of the lag-period of growth and could be only in part explained by increased cellular size during the same period of growth. The metabolic activities per unit-volume of the cultures, as measured by the above indices, reached maximal values near the end of the logarithmic period of growth and then decreased rapidly as the age of the cultures increased. Peptone or other complex media were employed in these studies. The present paper reports the influence of the concentration of lactic acid (sodium salt) on the oxygen-consumption, carbon dioxide-production and growth of Escherichia coli (K-12) in an inorganic medium of the following composition: NH4Cl, 5.0 gm.; Na2SO4, 2.0 gm.; MgSO4, 0.1 gm.; K2HPO4, 3.0 gm.; KH2PO4, 1.0 gm.; FeCla (0.5% solution), 0.5 ml.; CaCl2 (0.5% solution), 0.5 ml.; distilled water to make 1000 ml. Oxygen-consumption was measured by the usual Warburg technic and carbon dioxide-production by the technic described by Walker. All tests were carried out at 37.5°C. The influence of the concentration of lactic acid on the growth of Esch. coli, as measured with the aid of a photoelectric turbidimeter (Clifton), is illustrated in Fig. 1, A. Concentrations of sodium lactate greater than 0.1 N markedly inhibited the growth of this organism. The influence of 4 different initial concentrations of lactic acid on the oxygen-consumption of Esch. coli is illustrated in Fig. 1, B, in which cubic millimeters of oxygen consumed per minute per millimeter of the cultures is plotted against time in hours.