ortho-Phospho-L-serine (H-2 Psi, where Psi represents the serine-phosphato ion), a constituent of salivary proteins, seems to play an important role in the mineralization of teeth, To understand the basic mechanism of this interaction, the uptake of o-phospho-L-serine from relatively concentrated aqueous solutions (up to 100 mmol/L) onto synthetic hydroxyapatite was studied. Previous studies have shown that in the dilute concentration range(<12.5 mmol/L) the uptake followed a regular Langmuirian adsorption plot. At higher concentrations the uptake curve increased steeply, but no formation of a separate phase in the reacted apatite was discernible, either by optical or by scanning electron microscopy, The dissolution of apatite released phosphate and calcium ions into the solution in amounts linearly related to the uptake of serine with P/Psi = 1 and Ca/Psi = 2, The charge and mass balance of the reaction can be reconciled with the formation of the surface complex (shown within brackets) : Ca-10(OH)(2)(PO4)(6) + 6H(2) Psi --> [Ca-6(H Psi)(2)(HPO4)(2)(PO4)(2)] + 4Ca(2+) + 2H Psi(1-) + 2 Psi(2-) + 2H(2)PO(4)(1-) + 2H(2)O. The formation of two other surface complexes is possible; however, the complex shown above probably disrupts the apatite lattice the least, Traces of Ca Psi.H2O precipitate out from the filtrates of highly concentrated solutions after 6 days.