The lactating mammary gland synthesizes and secretes large amounts of phosphoproteins that mainly are associated with the casein fraction of milk. The free amino acids and inorganic phosphate of blood serve as building materials for casein, and the final product appears in milk as a colloidal-sized particle, the casein micelle. According to our present concept, the biosynthesis of casein occurs in two steps: synthesis of the polypeptide chain, followed by phosphate addition. Phosphate groups are transferred to the nascent casein by a protein kinase localized in the Golgi apparatus. The enzyme uses adenosine 5'-triphosphate as the phosphate donor and requires divalent cations. Neighboring amino acids may be important in determining which serine residues in casein are phosphorylated. This review discusses historical and current research on the phosphorylation of casein.