A dissolution cell has been constructed for use in measuring the rate of acid dissolution of test surfaces. Using discs of compressed hydroxyapatite in this specially constructed dissolution cell, their initial rate of dissolution in potassium acetate buffer (pH 4.65) can be determined with a coefficient of variation < 1 %. The reduction in the rate of dissolution of the hydroxyapatite discs after a 1-min topical application of fluoride solutions or slurries of fluoride toothpaste, having concentrations as low as 0.2 ppm F––, was easily demonstrable. This test was also sensitive enough to demonstrate fluoride activity after as little as 10 sec exposure of the discs to fluoride solutions. Enamel mosaics prepared from human caries-free teeth could also be employed in the dissolution cell. Results of 1 min pretreatment of these mosaics with monofluorophosphate toothpastes indicated activity of a similar order to that found using discs of pure hydroxyapat