The use of 13C-Fourier transform nuclear magnetic resonance (FT-NMR) has led to the observation that while 8 molecules of [2- 13C]-ALA are incorporated into vitamin B 12 in P. shermanii, [5- 13C]-ALA labels only seven of the carbon atoms of cyanocobalamin; i.e. one of the amino methyl groups of ALA is “lost” in the process. It has also been confirmed that seven of the methyl groups of vitamin B 12 are derived from [ 13CH 3]-enriched methionine and further that the chirality of the gem-dimethyl grouping at C 12 labeled with [ 13CH 3]-methionine is R. A soluble enzyme mixture from the 37,000 g or 100,000 g supernatant of disrupted cells of P. shermanii converts both 14C-labeled ALA and 14C-uro'gen III to cobyrinic acid, the simplest corrinoid material on the pathway to vitamin B 12 and the coenzyme, in presence of NADPH, Co 2+, Mg 2+, S-adenosylmethionine and glutathione. Multiply-labeled uro'gens ( 13C, 14C and 3H) have been used to show that incorporation takes place without randomization. A sequence for corrin synthesis from uro'gen III, involving as the first step decarboxylation of the ring-C acetic acid side chain, is proposed.