The work of French and Kahlenberg on gas-metal electrode potentials suggested that the potentials obtained at inert metal electrodes immersed in growing cultures of bacteria, and generally regarded as an oxidation-reduction phenomenon, might be of a similar type.This has been tested by passing air, hydrogen, nitrogen and carbon dioxide through sterile beef broth and observing the potentials developed at platinum, gold, and mercury electrodes. It is shown that air causes a rise in potential at all three electrodes; hydrogen causes a marked fall in potential at the platinum and a slight change at the gold and mercury electrodes; nitrogen and carbon dioxide cause little or no change in the potential at the three electrodes. These changes are in agreement with French and Kahlenberg's results except in the case of the mercury electrode which is shown to react chemically with the broth.