Human salivary secretions are supersaturated with respect to the calcium phosphate salts which form dental enamel, a property which provides important protection for the teeth. We previously proposed that statherin, a 43-residue phosphopeptide, plays a key role in this protective system by inhibiting or delaying potentially harmful precipitation of calcium phosphate salts in the salivary glands and mouth. The purpose of the present study was to determine if the concentrations of statherin in saliva, despite their wide normal range, are high enough to fulfill this function. Concentrations of statherin in stimulated human parotid saliva samples from 36 female and 32 male subjects, aged from 17 to 30 years, were determined by a single radial immunodiffusion method. Values found ranged from 3.0 to greater than 27.3 microM, with a mean value of 12.8 (S.D. +/- 5.46) microM. At concentrations below these values, statherin inhibited spontaneous precipitation of calcium phosphate salts from an assay system which was more supersaturated with respect to dicalcium phosphate dihydrate, and comparably supersaturated with respect to hydroxyapatite, than were human saliva samples. The inhibitory activities of five of the 65 stimulated parotid saliva samples assayed were greater than would be anticipated from their statherin concentrations. This unexplained discrepancy is not associated with the presence of the acidic proline-rich proteins in saliva, although these proteins also affect calcium phosphate precipitation. The results of this study show that statherin is present in stimulated human parotid saliva at concentrations and levels of activity which are consistent with its proposed biological function, and support the proposal that statherin plays a significant role in a system which provides a protective and reparative but stable environment for the teeth.