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A REPORT on reactions of the fluoride ion with synthetic hydroxyapatite was presented recently.1 It was shown that with relatively pure hydroxyapatite there is a gradual change in the type of reaction as the fluoride concentration increases. With a high ratio of liquid phase to solid (10 L. of solution to 2.5 Gm. of apatite), fluorapatite is formed at the lowest concentrations of F-and gradually, as the fluoride concentration increases beyond 100 ppm, more and more of the fluoride is converted to calcium fluoride. With a low ratio of liquid to solid (100 ml. of solution and 2.5 Gm. of apatite), the formation of fluorapatite is the chief reaction up to a concentration of 0.2 per cent fluoride; beyond this point, more and more calcium fluoride is formed.Several mechanisms for the reaction of fluoride with enamel, dentin, and bone have been suggested. Neuman, Neuman, Main, O'Leary, and Smith2 have shown that fluorapatite is formed below the solubility product of calcium fluoride. Falkenheim and Hodge,' and Volker, Hodge, Wilson, and Van Voorhis4 have shown that surface adsorption of fluoride takes place. The double decomposition of apatite with high levels of fluoride has been demonstrated by Gerould,5 Scott, Picard, and Wyckoff," and others. This paper presents the results of the extension of the work on synthetic hydroxyapatite to powdered enamel and dentin and an attempt to correlate these findings with those of other workers.