From a large number of posterior teeth with arrested natural carious lesions some 0.56 percent were found to have arrested approximal lesions with a superficial layer of calculus. The lesions were examined using quantitative microradiography for mineral content and by electron microprobe analysis for Ca, Na, Cl, Mg and P. The conditions in the plaque, which caused the formation of calculus, might also be expected to cause a remineralization of the lesions. However, the lesions examined were found to have a low mineral content in their body, and moreover, had a lower Na/Ca ratio and a higher Cl/Ca ratio than sound enamel; these are characteristics also found in carious lesions not covered with dental calculus. On the assumption that the lesions developed before any plaque calcification occurred, it is concluded that conditions which favour the calcification of plaque do not favour the remineralization of carious lesions in vivo. As only 0.5% of the carious lesions in extracted teeth were covered with calculus, it must be admitted that consecutive periods of lesion formation followed by calculus formation on the same tooth face are rare.