The behavior of ^<57>Co-B_<12> in rat intestinal loops in the absence of intrinsic factor (IF) has been investigated and the following results were obtained. 1) Small amounts of B_<12> placed in the loop are rendered non-dialyzable and unabsorbable within a short period of time. 2) There seems to be a rather constant amount of B_<12> binding capacity in loop washing at any time, although it is small, and it is produced and released into the lumen from the mucosa. There may be an equilibrium between production and degradation of the binder. 3) The loop is capable of inactivating a vast amount of B_<12>, if time is allowed, suggesting stability of binder conjugated with B_<12>. 4) In vitro systems the binding capacity of mucosal homogenates increases when incubated with Krebs-Henseleit bicarbonate glucose compared with physiological saline, and the same is true with mucosa cell suspensions. A chemical reaction involving glucose may be required in such increase of binding capacity.