Acid-etch biopsies were taken from the centre of the mesio-lingual, disto-lingual, disto-buccal and mesio-buccal cusps of non-carious erupted (n = 16, aged 18-28 years) and unerupted (n = 21, aged 18-28 years) teeth. The donor subjects had lived continuously in a low fluoride area (F water less than 0.1 parts/10(6] from birth, and had had no systemic fluoride supplementation. Tooth brushing, mouth rinsing and the application of sealers were the only fluoride-containing anti-caries programmes practised singly or in combination. Six successive acid-etchings were taken from each of the four cusps, and the fluoride levels in the etching solution were measured with an adapted, fluoride ion-selective electrode. Up to a depth of about 10 microns, unerupted enamel was etched significantly (p less than 0.05) more deeply than erupted enamel, which might be explained by changes in its composition. Up to a depth of approximately 10 microns, significantly more fluoride was found in erupted than in unerupted enamel. This difference may mainly be attributed to the effect of fluoride from the oral environment on erupted enamel. Fluoride concentrations, up to a depth of approximately 3 microns, were (p = 0.07) higher in the buccal cusps than in the lingual cusps of erupted molars, but not at the other etch depths. There were no significant differences (p greater than 0.10) between the buccal and lingual cusps of the unerupted molars at any depth. No significant correlations between enamel fluoride concentrations and age could be found either for erupted or unerupted third molars.