Four protein components which participate in the reversible glycine cleavage reaction were isolated from extract of Arthrobacter globiformis grown in a glycine medium. Two of those protein components (a pyridoxal phosphate enzyme designated as P-protein and a heat-stable acidic protein designated as H-protein) were homogeneous on SDS-disc electrophoresis. The apparent molecular weights of H-protein and P-protein were 20,000 and 270,000, respectively, as estimated by Sephadex gel filtration. Decarboxylation of the glycine carboxyl group absolutely required both P-protein and H-protein, and an H-protein-bound intermediate of glycine decarboxylation, which should be in the -CH2NH2 form, could be isolated. Incubation of the H-protein-bound intermediate with CO2 and P-protein yielded glycine, and incubation with T-protein (a THF-requiring enzyme) and THF gave methylene-THF. Hydrogen atoms at the a-position of glycine were not labilized throughout the whole process from glycine to methylene-THF. H-Protein contained a functional disulfide group, and decarboxylation of glycine appeared to be coupled with reductive cleavage of the disulfide group in H-protein, followed by the formation of an S-C bond between the H-protein-thiol and -CH2NH moiety of glycine. In the reverse reaction of glycine decarboxylation, in turn, the dithiol group of H-protein seems to be oxidized to disulfide. A tentative scheme is presented to explain the whole process of reversible glycine cleavage.