T o determine whether the molecular configuration of vitamin B₁₂ influences the attachment of intrinsic factor-vitamin B₁₂ complex to ileal microvillous membrane receptor sites, we have examined the kinetics of uptake of intrinsic factor-bound cyanocobalamin by brush borders and microvillous membranes isolated from guinea pig ileum, and have compared this uptake with that of intrinsic factor alone and with that of intrinsic factor complexed with various analogs of cyanocobalamin.We first studied the kinetics of binding of cyanocobalamin and other cobamides to human gastric intrinsic factor. The binding of cyanocobalamin showed saturation kinetics and, at relatively high concentrations of cyanocobalamin, a Scatchard plot of binding was linear. The dissociation constant for the intrinsic factor-cyanocobalamin complex was 0.066 nM. When the binding of various vitamin B₁₂ analogs to intrinsic factor was determined by competition experiments, the analogs could be separated into two categories: those with affinities similar to that of cyanocobalamin and those with affinities much lower than that of cyanocobalamin. The affinity of cyanocobalamin for intrinsic factor was not altered by various substitutions at the -CN position, while removal of a single amido group on the corrin ring of substitution of the dimethylbenzimidazole base greatly reduced affinity. Removal of the base totally abolished binding. These findings, confirming those reported by others, are consistent with the concept that the cyanocobalamin molecule fits into a “pocket” in the intrinsic factor molecule, with the nucleotide base facing inward and the -CN side of the planar corrin ring facing outward.We then investigated the attachment of intrinsic factor-bound cyanocobalamin to ileal receptor. Attachment to microvillous membranes showed saturation kinetics with a dissociation constant of 0.25 nM. Attachment was rapid and was 70% complete within 5 min; the second-order rate constant for attachment was 1.3 × 10⁶ M^-1 s^-1. The half-time for dissociation of intrinsic factor-bound cyanocobalamin from the ileal receptor was approximately 35 min. Free intrinsic factor inhibited the attachment of intrinsic factor-bound cyanocobalamin, but the rate of attachment of free intrinsic factor was slower than that of intrinsic factor bound to cyanocobalamin. When intrinsic factor was complexed with various analogs of cyanocobalamin, the affinities of these complexes for ileal microvillous membranes were similar to that of intrinsic factor-bound cyanocobalamin. These findings suggest that the molecular configuration of vitamin B₁₂ is not a major determinant in the interaction between intrinsic factor-bound vitamin B₁₂ and its ileal receptor site.