The stability of calcium phosphate clusters approximately 0.8 nm in diameter, which were found in a simulated body fluid in a previous work, was investigated in the system CaCl2-H3PO4-KCl-H2O using an intensity-enhanced dynamic light-scattering technique at 25 degrees C. It was shown that tris(hydroxymethyl)aminomethane, which was used as a buffer in the previous study, had no effect on the formation of the clusters. The amount of clusters decreased with decreasing supersaturation of the solution with respect to hydroxyapatite. Under constant supersaturation with respect to hydroxyapatite, the amount of the clusters reached a maximum in solutions with Ca/P molar ratios ranging from 1 to 8.