Abstract
Based on some observed properties of the B12-intrinsic factor complex, an assay procedure is described for determining serum Vit. B12 levels using isotopically labeled Vit. B12 and intrinsic factor. Eighty-three-112% of crystalline Vit. B12 added to serum was recovered in the assay. Concentrations of Vit. B-12 deficient, and leukemic sera were in the ranges, 104-328 [mu][mu]/ml, 0-72 [mu][mu]g/ml and over 1000 [mu][mu]g/ml respectively.