The differential inorganic composition of the dentine of 14 sound, active and arrested carious teeth was investigated by microchemical analysis. Disc-shaped dentine samples, 150 μm in diameter, were punched from previously microradiographed, pianoparallel tooth sections, the calcium and phosphorus content of each sample was estimated and the results were expressed on a weight per unit volume basis. Progressing lesions showed a continuous loss from the demineralisation front to the surface of up to 0.75 of the original mineral with a tendency towards preferential removal of calcium in the more active lesions. The arrested lesions generally showed similar changes in the deeper areas but displayed a radiopaque surface zone containing a calcium-rich mineral producing very high surface Ca:P ratios and thought to be derived from the oral fluids
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