Abstract Exposure of both small carious lesions and artificial caries-like lesions to a synthetic calcifying fluid in vitro produced a significant degree of remineralization of the lesion. Changing the calcium concentration of the calcifying fluid had a marked effect on the degree of remineralization produced. Only the apatite phase was supersaturated in the 1 mM Ca calcifying fluid while the 3 mM Ca calcifying fluid was supersaturated with respect to several more acidic calcium phosphate phases. With the 3 mM Ca calcifying fluid the higher supersaturation allowed the formation of precursor phases which blocked surface pores limiting penetration. This was confirmed by the appearance of clusters of plate-like crystals in the immediate surface layer, a feature not found with the 1 mM Ca calcifying fluid. With the 3 mM Ca calcifying fluid the mean reduction in area of the body of the lesion was 20 per cent, brought about by a mean increase in orientated mineral of 17%. When the 1 mM Ca calcifying fluid was employed, remineralization occured throughout the entire depth of the lesion. Under these conditions, a mean reduction of 69 per cent in area of the body of the lesion occurred with a mean increase of 40% in orientated mineral. Scanning electron microscopy showed that crystal diameters for sound enamel were in the range 35–40 nm. In the body of the lesion crystal diameters were reduced and found to be in the range 10–30 nm. In lesions remineralized with the high Ca calcifying fluid crystal diameters of 50 nm were found. When the low Ca calcifying fluid was used, remineralized lesions showed crystal diameters in the range 50–150 nm with a small number having diameters of 200 nm.