Publisher Summary This chapter describes immobilized derivatives of vitamin B12 coenzyme and its analogs. The immobilized derivatives of cobalamins are useful not only for purification of cobalamin-dependent enzymes or cobalamin-binding proteins, but also for basic studies on biospecific interactions between cobalamins and apoenzymes or cobalamin binders. Vitamin B12 coenzyme (adenosylcobalamin) is an organocobalt compound that has a very complicated structure. The molecule is composed of the three major parts; a corrin ring (equatorial quadridentate), a lower nucleotide ligand moiety (axial fifth ligand), and an upper nucleoside ligand moiety. Immobilization of corrinoids is accomplished by either (a) reaction of ω-aminoalkyl-Sepharose with the carboxylic acid derivatives of corrinoids in the presence of 1-ethyl-3 (3-dimethylaminopropyl) carbodiimide; or (b) reaction of cyanogen bromide-activated Sepharose with the amino derivatives of Corrinoids. This chapter describes methods for preparing several new corrinoid affinity adsorbents and also briefly describes their application for a basic study of the interaction of the cobalamin coenzyme with the apoenzyme of cobalamin-dependent diol dehydrase.