The present invention relates to assay methods for the determination of cobalamin or vitamin B.sub.12 in a body fluid and in particular to assay methods for the metabolically active pool of cobalamin, comprising contacting a cell free sample of a body fluid with an immobilised or immobilizable specific binding ligand for TC II or holo TC II, separating a ligand bound fraction from a non-ligand bound fraction and measuring the holo-TC II or TC-II bound cobalamin content therein.