In order to obtain information about the chemical reactivity of localized areas within natural enamel caries, thin ground sections prepared from white spot lesions were subjected to HCl solutions with 0.01, 0.05, and 0.1 N concentrations. Microradiographic examination combined by densitometric analysis demonstrated that the surface layer covering demineralized lesions remained relatively stable against acid attacks and that, despite the original lower degree of mineralization, its microradiographic density became higher than that of the underlying normal enamel during acid-dissolution. Another interesting finding was that in the experiment using the most diluted HCl solution, a radiopaque zone became noticeable at the advancing front of the lesions in the course of dissolution. X-ray microbeam diffraction analysis of these sections showed that no apparent alterations occurred in mineral structure of enamel with acid dissolution, except for the decrease in crystallinity of enamel apatite. Based on the results obtained here, one can expect that the surface layer and the interior part of natural carious lesions possess a high degree of resistance to acid attacks, relative to the underlying normal enamel.