Thirty-five erupted and 15 unerupted caries-free teeth were painted with varnish leaving ‘windows’ of enamel on buccal and lingual surfaces. Each tooth was slit in half longitudinally and one half, selected at random, was stored in a calcifying fluid whilst the control half was placed in distilled water. The calcifying fluid was used in the range pH 6.8–7.4. Tooth halves were exposed to the fluids from 1–24 weeks, the solutions being renewed every other day. After exposure, the ‘windows’ were brushed with water using an automatic toothbrush. After this, the tooth halves were placed in acidified gelatin gels. Both halves of each tooth were exposed to identical conditions of attack and exposure time. After exposure to the gels from 8–24 weeks, longitudinal sections through the lesions were examined with the polarizing microscope. Results of qualitative and quantitative imbibition studies indicate differences in histology, rate of attack and internal pore volume between control and experimental lesions. The controls were more severely affected than the precalcified experimental halves with respect to these criteria. The ‘beneficial’ effect of the calcifying fluid was more marked with unerupted than with erupte