The surface of a poly(lactic acid) composite (CCPC) containing calcium carbonate was reported to show apatite formation in simulated body fluid (SBF) within 24 h at 37 degrees C. In the present work, zeta potentials of CCPC were examined to determine the rapid apatite formation mechanism at an early stage after soaking in SBF. The zeta potentials of CCPC showed negatively charged values in SBF. The potential decreased immediately after soaking, and subsequently increased during 1-6 h of soaking. After 6-9 h of soaking, no change in zeta potentials was observed. Laser Raman spectroscopy results suggested that the change in the zeta potential is closely related to the amount of Ca2+-coordinated carboxy groups on the CCPC surface. The concentration of phosphate ion in SBF decreased after 6 h. Apatite formation was suggested to begin after 6-9 h of soaking via Ca2+ coordination to carboxy groups and the subsequent adsorption of phosphate ions.