Abstract Extensive evidence is presented that incubation of reduced Escherichia coli 13 vitamin-B 12 transmethylase with N 5-methyl- 14C-tetrahydrofolate (plus unlabeled S-adenosylmethionine), with methyl- 14C- S-adenosyImethionine alone, or with methyl- 14C iodide resulted in the formation of a methyl- 11 C-cobalamin enzyme. Under optimal conditions N 5-methyl- 14 C-tetrahydrofolate and methyl- 14C-S-adenosylmethionirie yielded 0.5 equivalent and 1 equivalent, respectively, of melhyl- 14C-cobalamin enzyme. Unlabeled N 5-methyletrahydrofolate decreased methyl- 14C-B 12 enzyme formation with methyl- 14C- S-adenosylmethionine by 10-fold. Methyl- 14C-cobalamin enzyme formation was accompanied by changes in the absorption spectrum of the initial transmethylase and produced a spectrum that displayed the characteristic of free methylcobalamin. Upon the addition of homocysteine to a methyl- 14C-cobalamin enzyme, methionine- 14CH 3 was obtained and the spectrum reverted back to one which was similar to that for the original enzyme. Regardless of the methyl- 14C group donor, the methyl- 14C-cobalamin enzyme which formed was stable to light at 0 ° unless the protein solution was acidified to pH 2.