Metal-free, zinc, copper, and rhodium analogues of vitamin B12 were synthesized to further characterize structural requirements for the binding to human intrinsic factor, transcobalamin I, and transcobalamin II. Binding affinities of the various analogues were studied by competition against cyano[57Co]cobalamin. When albumin-coated charcoal was used for the separation of free and bound corrinoids, the relative 50% inhibition indexes were determined. The influence of metal substitution was similar among the three binding proteins. For analogues with a strong coordinative linkage between the heterocyclic base and the central metal ion, similar to that with cobalt (e.g., zincobalamin and cyanorhodibalamin), the indexes range from 0.65 to 2.35 for all three binding proteins. Analogues in which coordination is impossible (hydrogenobalamin and dicyanorhodibalamin) exhibit markedly reduced binding with indexes between 10 and 160. Cupribalamin shows 50% inhibition indexes ranging from 2.3 to 5.0, thus suggesting a weak coordinative bond between the copper ion and the 5,6-dimethylbenzimidazole moiety. These results emphasize the importance of the coordinative linkage between the central metal ion and the nucleotide moiety for optimal recognition by vitamin B12 binding proteins.