The use of 13 C-Fourier transform nuclear magnetic resonance (F.t.-n.m.r.) has led to the observation that while 8 molecules of [2- 13 C] ALA are incorporated into vitamin B 12 in P. shermanii , [5- 13 C]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 B 12 are derived from 13 CH 3 -enriched methionine and further that the chirality of the gemdimethyl grouping at C 12 labelled with [ 13 CH 3 ]methionine is R. A soluble enzyme mixture from the 37 000 or 100 000 g supernatant of disrupted cells of P. shermanii converts both 14 C-labelled ALA and [ 14 C]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-labelled uro’gens ( 13 C, 14 C and 3 H) have been used to show that incorporation takes place without randomization. A sequence for corrin synthesis from uro’gen III is presented.