The object of this study was primarily to test the possible caries inhibiting effect of hexafluorostannate on human dentine. By employing the gold plate technique it was also intended to establish the feasibility of this method for studying dentinal caries. By means of grinding, the greater part of the buccal enamel was removed from seven premolars in situ. One half of the experimental area, consisting of abraded enamel and dentine, was given a 5 min topical application of sodium hexafluorostannate, having an initial fluoride concentration of 0.3 m and a pH of 2.35. Caries was induced for 16 to 20 days prior to extraction. The teeth were inspected in a dissecting microscope, photographed, sectioned, and subjected to microradiography. In most specimens a distinct difference was observed between the test and the control area, suggesting a caries protective effect of the solution in the dentine as well as in the enamel. These observations were confirmed by microradiography, which showed subsurface demineralization with a relatively well preserved surface layer. The demineralization revealed a pattern corresponding to dentinal tubules. Apparently the spread of the lesion was unaffected by the presence of interglobular spaces.