The residues of 5 samples of powdered human enamel, each subjected to 5 sequential equilibrations at 37 degrees C with either 17 or 4 mmol/l phosphoric acid, were examined microscopically. With 17 mmol/l acid, both brushite and monetite were found after 1 equilibration but, after further equilibrations, brushite was no longer present and the abundance of monetite crystals increased. Formation of monetite probably contributed to the lower metastability of this system compared to similar low-pH systems at 25 degrees C, where monetite does not form. Neither brushite nor monetite were present after equilibration with 4 mmol/l acid. Whitlockite was identified by transmission electron microscopy and electron diffraction in all residues. In the 4 mmol/l systems, the ionic activity product (IMWH) for magnesium whitlockite, Ca9Mg(HPO4)(PO4)6, became constant after 1-3 equilibrations, at a mean value of 3.6 (+/-0.51 SE).10(-105), which may reflect saturation with respect to this solid. For the 17 mmol/l systems, higher values of IMWH, and supersaturation with respect to monetite, were interpreted as evidence for persistent metastability due to slow crystal growth of whitlockite and monetite. It is concluded that neither brushite nor monetite are likely to form within carious lesions, but the results are consistent with the known association of whitlockite with caries.