A method of determining the vitamin D content of the blood serum is desirable in the diagnosis and treatment of rickets and other disturbances of mineral metabolism and maladies of the bones. Clinical and roentgenologic signs are not always sufficient. Chemical methods may be helpful, the best known and most widely employed being estimation of the inorganic phosphorus content of the blood. Variations in the inorganic phosphorus content and the blood phosphatase level (the determination of which has recently been recommended) are not specific for rickets, since these changes may result from a variety of pathologic conditions. A direct estimation of the vitamin D content of the blood serum would therefore be an additional diagnostic asset. Attempts to accomplish this have been made by various investigators. Unfortunately, the fact that the physical and the chemical properties of vitamin D are not well defined renders difficult its determination by such methods.