The amounts of firmly and loosely bound fluoride were determined in sound enamel of unerupted and erupted teeth which had been exposed in vivo for 1-16 yr to brushing at least once a day, and occasionally to mouth rinsing and the application of sealers. Enamel was sampled by an acid-etch procedure, and the fluoride levels were measured with an adapted fluoride ion-selective electrode. Unerupted enamel was etched significantly (p less than 0.05) deeper than erupted enamel up to a depth of at least 8 microns. Significant differences (p less than 0.05) were found between the mean enamel fluoride concentrations of unwashed and alkali-washed, erupted teeth up to a depth of at least 3 microns and also between unwashed or washed, erupted versus unwashed or washed, unerupted teeth. At a depth of 3 microns, the fluoride treatments of enamel had increased the total amount of fluoride by approx. 78% of which approx. 53% was loosely bound fluoride (like CaF2) and 47% firmly bound (like fluorapatite). No increase in sound enamel fluoride as a result of topical treatments over a period of up to 16 yr could be found at a level deeper than 20 microns.