When 18F is used to measure the uptake of fluoride from dilute fluoride solutions by hydroxyapatite or enamel, it is probable that the observed uptake of 18F does not give a true estimate of the uptake of stable fluoride because some 18F in the solution phase exchanges iso-ionically with stable fluoride in the solid phase. To demonstrate the extent of this iso-ionic exchange radioactivity measurements and chemical analyses for 18F and stable fluoride were made upon the same samples. When the hydroxyapatite originally contained very little fluoride, the uptake of 18F was exactly comparable to the uptake of stable fluoride determined chemically. However, powdered hydroxyapatite samples, pretreated to raise their fluoride content, and powdered enamel samples containing 172 ppm F, showed little or no uptake of stable fluoride but a significant uptake of fluoride as 18F. Under the conditions of the experiment iso-ionic exchange of radiofluoride for stable fluoride must have been responsible for the observed uptake of 18F. It would appear therefore that 18F should not be used in the measurement of fluoride uptake by enamel from dilute fluoride solutions. The uptake of 18F is theoretically predictable in terms of the stable fluoride concentrations in the solutions, and the number of replaceable sites in the hydroxyapatite. This theory is in reasonable accord with experimental behaviour. Uptake by powdered enamel, which is sensibly at equilibrium, is completely different in character from uptake by intact enamel where kinetic considerations apply. Hence the behaviour of intact enamel is not in general predictable from data obtained on powdered enamel.