The effects of the pH of the culture medium on the biosynthesis of NADH-rubredoxin oxidoreductase byClostridium acetobutylicum, strain ATCC 824, have been studied. Between pH 6.8 and pH 4.2, the rate of enzyme formation fluctuated in the proportions of 1–50 respectively. The induction of the rubredoxin reductase synthesis, normally observed at pH 4.3, was stopped immediately after the addition of rifampicin. It is suggested that NADH-rubredoxin reductase could play a role in some deacidification mechanism in relation to proton transport.