When the growth medium of Clostridium acidiurici was supplemented with trace metals, glycine synthase and glycine-CO2 exchange activities in cell-free extracts were found to increase significantly. The glycine-CO2 exchange system was purified and shown to consist of a heat-labile component and a heat-stable component. By gel filtration, heat-labile component had an estimated native Mr = 230,000 and contained two subunits of Mr = 65,000 and 58,000 on sodium dodecyl sulfate-polyacrylamide gels, indicating an alpha 2 beta 2 tetramer. Heat-stable component had an estimated Mr = 20,000 and could not be replaced by lipoic acid in reaction mixtures. Pyridoxal phosphate was not bound to either of the purified components but was essential for glycine-CO2 exchange. By spectral analysis, heat-labile component was shown to interact with pyridoxal phosphate and that reductant influenced this interaction.