The elimination of radioactivity was followed for periods up to 30 days after parenteral administration of 0.5 [mu]g doses of Co56 labeled B12. During the first few days after injection there was a transient high urinary excretion. From the fourth day on, the excretion pattern was characterized by a steady fecal output of radioactivity, whereas the urinary radioactivity was low. Studies on bile, obtained by duodenal lavage or by postoperative drainage, indicated that 3-4 times more vitamin is excreted with the bile than with the feces per day. About 0.3% of the radiovitamin contained in the body were excreted daily by the fecal route. If the labeled vitamin were uniformly mixed with the body B12 stores, the normal loss of tissue B12 could be estimated to be of the order of magnitude 12 [mu]g/day. This is considerably more than hitherto assumed. In man B12, or its break-down products, appears to be excreted mainly by the bile-feces route and to be subject to an enterophepatic circulation.