The purpose of this study is to investigate the mechanism by which (NH4) 2TiF6 solution produces its cariostatic effect by means of comparing the artificial caries lesion formation on enamel after treatment with (NH4) 2TiF6 and APF solutions and determing the reaction products after treatment with (NH4) 2TiF6 solution.Human intact enamel was demineralized (Moreno, 1974) after treatment with (NH4) 2TiF6 and APF solution (9000ppmF-, pH 3.4) for 1hr following a synthetic saliva wash (Birkeland, 1973) for 24hr. The artificial caries-like lesions were investigated by polarized light microscopy, contact microradiography, SEM observation, EPMA, and X-ray diffractometry. A polarized light photomicrograph showed that control enamel surfaces had well-developed lesions, whereas the experimental surfaces had smaller lesions. After saliva wash and the formation of caries-like lesions, the SEM image showed that surfaces treated with (NH4) 2TiF6 solution has a tenacious coating. EPMA revealed the distribution of titanium and fluoride in the caries-like lesions of enamel treated with (NH4) 2TiF6 solution.Human enamel powder (passed through 200 mesh) was treated with (NH4) 2TiF6 solution (79.0 and 8.8 mM, pH 3.4) for 3, 5, 10, 20, 30, 40 and 60 min. at 37°C. Each solution was filtered and analyzed for calcium, phosphate, titanium and fluoride.Gibbs' free energy was computed with respect to CaF2, Fap and Hap in the experimental solutions. Two mol HClO4 acid-insoluble product was determined with an analytical electron microscope. Organically extructed enamel powder was prepared to investigate the relationship between glaze and organic materials.In the experimental solution (79.0 mM) free F- increased for the first 10 min. and decreased slowly, and Ti4+ decreased for the first 10 min. and then kept almost constant. This result suggests that F- and Ti4+ are disassociated from [TiF6]2-and react with enamel independently. ΔG showed that experimental solutions were supersatulated with respect to CaF2 and Fap but not Hap. Acid-insoluble product, which was not formed in organically extructed enamel, contained titanium and phosphorus.Human enamel treated with (NH4) 2TiF6 solutions formed CaF2, Fap and glaze-like product containing titanium and phosphorus and organic materials. The glaze-like product has strong acid resistance and it may act a major inhibitor of caries formation.