A new radioisotope dilution assay for vitamin B12-intrinsic factor complex is described. The method is based on the use of the binding type intrinsic factor antibody (the binding reagent), which when combined with the intrinsic factor-vitamin B12 complex (labelled ligand), is quantitatively adsorbed onto zirconium phosphate gel at pH 6.25. The new assay has been shown to provide a measure of intrinsic factor comparable with other intrinsic factor assays, but it has the important advantage of being able to measure the unlabelled vitamin B12-intrinsic factor complex (unlabelled ligand), and will, therefore, be valuable in the study of physiological events in the gastrointestinal tract. During the study, it was found that there is some evidence for a least two types of binding intrinsic factor antibody: One which combines preferentially with the intrinsic factor-vitamin B12 complex and one which combines equally well with this complex or with free intrinsic factor.