“…It is now well documented that free amino acids of blood are used for the synthesis of casein in the mammary tissue (Barry, 1961). It is also known that phosphorus that is bound by ester linkage to the hydroxyl group ofthe serine of casein (Lipmann, 1933;Burnett & Kennedy, 1954) is supplied by inorganic phosphorus of blood (Aten & Hevesy, 1938;Barry, 1952;Kleiber & Luick, 1956). However, the mechanism by which phosphorus is incorporated into the protein molecule in the mammary gland has not yet been clearly delineated.…”