Proline-rich proteins are major components of parotid and submandibular saliva in humans as well as other animals. They can be divided into acidic, basic and glycosylated proteins. The primary structure of the acidic proline-rich proteins is unique and shows that the proteins do not belong to any known family of proteins. The proline-rich proteins are apparently synthesized the acinar cells of the salivary glands and their phenotypic expression is under complex genetic control. The acidic proline-rich proteins will bind calcium with a strength which indicates that they may be important in maintaining the concentration of ionic calcium in saliva. Moreover they can inhibit formation of hydroxyapatite, whereby growth of hydroxyapatite crystals on the tooth surface in vivo may be avoided. Both of these activities as well as the binding site for hydroxyapatite are located in the N-terminal proline-poor part of the protein. Little is known about the functions of the glycosylated and basic proline-rich proteins.