The purpose of this study was to improve the sensitivity of the biopsy method developed by Brudevold et al. Enamel surfaces were abraded by means of rubber cups and silicon carbide. The slurry was collected by the lip of the cup, which was cut off and transferred to a plastic test tube. The material was dissolved in perchloric acid and buffered by sodium citrate. Fluoride was determined by a combination fluoride activity electrode and calcium by atomic absorption spectrophotometry. The method permits determination of the fluoride content in approximately 0.2-μm thick layers within single tooth surfaces. In two selected groups of 207 children (7–16 years of age) residing in high and low fluoride areas, the fluoride concentration in the enamel surface ranged from 1,000 to 6,000 ppm. In 33 children two successive layers were sampled, each being approximately 0.5 μm thick. In the first layer the fluoride concentration ranged from 2,000 to 5,800 ppm and in the second from 1,500 to 4,800 ppm.