The effects of the proline-rich phosphoproteins (PRP) on the rate of precipitation of CaCO3 from a CaCO3-supersaturated solution were examined by recording the absorbance at 570 nm and the pH, when 20 mM CaCl2 was added to 20 mM NaHCO3, in the presence or absence of proteins. The PRP suppressed CaCO3 nucleation and exhibited inhibitory effects on CaCO3 precipitation under those conditions and under stimulated physiological conditions (final concentration of Ca2+ was 2 mM and that of HCO-3 60 mM, pH was 8.0 at 37 degrees C). PRP may be of biological significance in maintaining homeostasis of the buffering system of saliva, which is mainly composed of bicarbonate, and in preventing the formation of stones consisting of CaCO3 in the salivary ducts.