At least two separate proteins with calcium-binding activity have been detected in the human duodenum. One of these proteins migrates rapidly during electrophoresis, has a molecular weight of about 20,000 (similar to the vitamin D-sensitive protein in the chick or monkey), and contains no detectable carbohydrate or phosphate. Whether it is vitamin D-dependent is as yet uncertain. The activity of the calcium-binding protein has been found to vary linearly as a function of the background binding activity of the resin used in the assay. Accurate determinations of calcium-binding activity depend, therefore, upon correction of all values to correspond to a standard background binding activity.