Plaque grown on terylene gauze in the mouths of 5 volunteers was treated with a mineralizing solution intermittently for 4 days to deposit fluorhydroxyapatite. Control plaque grown simultaneously was not treated. Sixteen hours after the last treatment, plaque gauzes were incubated in 0.28 M glucose under N2 at 37 degrees C. The mean pH, reached by the 5 mineralized plaques after 30 min, (4.78) was significantly higher than the mean pH reached by control plaques (4.13), a difference that was due neither to unequal microbial mass nor to unequal acid concentrations. Acid neutralization following the dissolution of apatite was probably mainly responsible for the pH differences although a small antiglycolytic effect from leached F could not be ruled out. Mineralized plaque lost on average 24 per cent of its Ca, 25 per cent of its P and 16 per cent of its F, resulting in 0.868 mM Ca, 0.676 mM P and 0.075 mM F in the supernatant. Test plaque fluid was saturated with respect to fluorapatite and only moderately undersaturated with respect to hydroxyapatite at the end of the incubation period; this could explain the pronounced caries-protective effect of plaque fluorhydroxyapatite shown previously.