Vitamin B12 is synthesized by only certain bacteria but not by plants. Thus, foods derived from animals, namely meat, milk, fish, and shellfish, are major dietary sources of vitamin B12. Intestinal bacteria have the ability to synthesize various corrinoid compounds carrying different base moieties in the lower ligand. Thus, animal manures and human feces contain substantial amounts of various cobamides with different bases. When food corrinoids were purified and characterized, certain food items unexpectedly contained pseudovitamin B12, in which adenine replaces 5,6- dimethylbenzimidazole as the base. Cyanobacteria used as human health supplements provide substantial amounts of pseudovitamin B12, which functions as a cofactor for cobalamin-dependent methionine synthase in these organisms. Oral administration of pseudovitamin B12 to mammals suggests that this cobamide does not act as a vitamin B12 antagonist in the gastrointestinal absorption of vitamin B12 and cobalamindependent enzyme systems. Furthermore, vitamin B12 is converted into an inactive analogue vitamin B12[c-lactone] by the treatment with chloramine-T, which is used as a disinfectant. The unnatural vitamin B12 analogue was found in certain dried mushroom fruiting bodies.