In one of the less appetizing but more brilliant experiments in the field of vitamin B-12 metabolism in the 50s, Sheila Callendar (7) in England delineated that colon bacteria make large amounts of vitamin B-12. Although the bacterial vitamin B-12 is not absorbed through the colon, it is active for humans. Callendar studied vegan volunteers who had vitamin B-12 deficiency characterized by classic megaloblastic anemia. She collected 24-h stools, made water extracts of them, and fed the extract to the patients, thereby curing their vitamin B-12 deficiency. This experiment demonstrated clearly that 1) colon bacteria of vegans make enough vitamin B-12 to cure vitamin B-12 deficiency, 2) the vitamin B-12 is not absorbed through the colon wall, and 3) if given by mouth, it is absorbed primarily in the small bowel.