Normal meat infusion broth at pH 7 had a potential nearly equal to that of the saturated calomel electrode. In a vacuum inhibiting growth of B. megatherium ([less than or equal to].10 mm. air pressure), this potential drops to [long dash]0.160 volt or below. If Na2SO3 is added to meat infusion broth, growth is inhibited when the conc. of sulphite reaches 057%. The potential of this medium is nearly [long dash]0.250 volt in the air and does not change in vacuo. When ferric am-monium citrate is added to nutrient broth in a conc. of 0.5%, growth takes place readily. In the air, the potential of such a medium is over +0.100 volt, and about [long dash] 0.040 volt under a vacuum that inhibits growth of B. megatherium. Even at the relatively high potential of [long dash]0.040 volt, the organism fails to grow. The O contents of the above media, after standing for 24 hrs. at 30[degree] C, were, in cc. of O in 20-cc. samples, as follows: meat infusion broth at pH 7, 0.07 cc; meat infusion broth + 0.27% Na2SO3, 0.00 cc; meat extract broth + 0.5% ferric ammonium citrate, 0.12 cc. These results show that the limiting factor for growth of B. megatherium in vacuo is the O content, and not the oxidation-reduction potential, of the culture medium.