1) A vitamin B_<12>-producing bacterium, Protaminobacter ruber formed a considerable amount of B_<12>-dependent apo-methionine synthetase when grown on the cobalt-deficient medium containing 1,2-propanediol as carbon source and supplemented with sodium succinate and serine. Since more apo-enzyme was synthesized at early phases of the bacterial growth, the enzyme for affinity chromatography was extracted from such early phase cells. 2) With the affinity chromatography using a B_<12>-Sepharose, methionine synthetase from P. ruber was successfully purified in good yield and in high purity (specific activity: ca. 30 nmol/mg/min). 3) The behaviors in the affinity chromatography seem to support the view that the methionine synthetase from P. ruber catalyzed N^5-CH_3-THF: homocysteine transmethylation and CH_3-B_<12>: homocysteine transmethylation.