A method is described for the micro determination of carbonate in 50-µg particles of human dental enamel. The technique is rapid and more sensitive than previous procedures. Amounts of carbonate from 0·5 to 3·0 µg have been determined with an accuracy of 4 to 7 per cent. (standard deviation). Carbon dioxide is liberated by dissolving enamel particles in acid. The gas forms a single bubble, flattened into a 100-µ thick disc between the parallel glass surfaces of a Neubauer haemocytometer. The area of the flattened bubble is measured and the volume of gas calculated. By using this technique in combination with a recently developed sampling procedure, it has proved possible to measure variations in carbonate concentration within thin sections of dental enamel.