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A b s a Recent discoveries in the biochemistry and biosynthesis of vitamin B12 are reviewed. The mechanism of methionine synthase involves a remarkable interchange of the DMBI moiety with a histidine group of the enzyme. It has been found that there are two distinct pathways to vitamin B 12 in nature, depending on whether the B12 producing organism is aerobic or anaerobic. The multi-enzyme synthesis of the corrin, hydrogenobyrinic acid, has been accomplished. An upsurge of research activity has uncovered some remarkable features of the mechanism of action and of the biochemical origin of vitamin B12, a molecule which has been aptly described as "Nature's most beautiful cofactor " (1). The basis of this renaissance is undoubtedly the acquisition, by cloning, sequencing and expression, of the genes required for the biosynthetic pathway and for several coenzyme B12- mediated processes