The growth of Clostridium sticklandii on the substrate pair L-alanine-L-proline (reductant-oxidant each 40 mM) in a medium containing 2 g/l yeast extract was completely inhibited by equimolar amounts of glycine, although glycine itself should be used as oxidant by the cells. The effect of glycine was the same, whether L-alanine, L-arginine, or L-serine wwere used as reductants. Performance of the growth experiments in media of high osmolarity excluded the possibility that the inhibition effected by glycine was caused by the synthesis of defective cell wall peptidoglycan. In cell-free extracts an inhibition of L-proline reduction by glycine was observed that did not belong to anyone of the known types of kinetic inhibition. It depended upon the presence of a functioning glycine-reducing enzyme system, besides glycine itself, and was lost after the purification of D-proline reductase. It was concluded from these results that a protein, besides glycine, participated in the inhibition of L-proline reduction. The regulatory implications of the inhibition for the energy metabolism of C. sticklandii are discussed.