On clinically sound buccal and lingual surfaces of 130 extracted teeth, class V cavities were prepared and filled. In some cases previously filled teeth were used. In vitro caries around the fillings was produced by immersing the teeth in an acid-gelatin system, the pH of which was held within the values 4.0–5.2 by means of lactic acid. Duration of the experiments varied within the range of 5–200 days. A number of teeth which were filled, but not exposed to the acid gelatin, were used as controls. Ground sections in series were prepared through the teeth with the fillings in situ and studied by ordinary transmitted and incident light, polarized light, ultraviolet light, and microradiography. The lesions produced consisted of two regions, the outer lesion and the cavity wall lesion. In the enamel the shape of the former and its relation to the latter seemed to be influenced by the angle made by the cavity wall with the prisms. The outer lesion developed by an action on the outer surface of the enamel or cementum and showed the features of a primary attack. Usually the wall lesion was a narrow subsurface defect in the enamel or the enamel and the dentine of the cavity wall. It represented an initial stage of the caries process and gradually encompassed the whole filling without penetrating deeply into the tissue. The development of the wall lesion was determined by the amalgam-dental tissue interface into which hydrogen ions must have diffused from the acidified gelatin. The demineralization seemed to have acted perpendicularly to the surface of the cavity wall in which reprecipitation had taken place in a few cases.