An electron microprobe analyser associated with a scanning electronic microscope was used to measure Ca, P and Mg contents of the human dental root hard tissues, before and after a citric acid treatment (pH = 1). The measurements were made on transverse sections through the cervical 1/3 of the molar roots. The measurements were performed at the following 8 levels: the internal cementum, the cementum-dentine junction, cementum-related dentine, 4 external dentine levels located at 220 microns, 420 microns, 620 microns and 820 microns from the cementum-dentine junction, and finally the juxta-pulpal dentine. After the citric acid treatment, the losses in Ca and P, but not in Mg, varied significantly with the level; an acid-resistant dentine layer of approximately 600 microns was found under the cementum-dentine junction. An increase in the Ca/P ratio was also observed in this layer. Since this external dentine zone is less demineralized by the citric acid than the underlying dentine, the collagen matrix in this region may also be less exposed. These differences in the response to citric acid that depend on the distance from the root surface may explain the unpredictability of periodontal healing after citric acid treatment of diseased root surfaces, because the amount of tissue removed during root planing is not constant.