A radioimmunoassay for the R-type binders of cobalamin

A radioimmunoassay of R-type binders of cobalmin was devised and tested. The assay incorporated an antibody against purified human salivary R binder as the binding reagent. The labeled ligand was cyano[57Co]cobalamin bound to the R binder of pooled human saliva. The standard source of unlabeled ligand was also from human saliva of known R binder content. The assay was responsive to R binders of several sources, to either pure R or that of crude sources and equally to R binder saturated or unsaturated with cobalamin. It was not responsive to transcobalamin II. The assay was reproducible and reliable.