AMONG the many facts which have been discovered about dental caries two, when considered together, are suggestive. First, there is the widespread observation that certain individuals or communities are, or have been, free of dental caries. A simple change in living conditions or diet has on a number of occasions been found to render these teeth susceptible to caries. One cause of the immunity is known to be fluorine, but it is also known that it is not the only one. The second is the initial carious attack on the enamel. Scott and Albright¹, followed by others, have shown that an early stage in the carious lesion, before the arrival of bacteria, is the degradation and removal of prism cores, leaving the walls with their embedded crystallites intact. (The appearance of this altered enamel in the electron microscope is similar to that of the section shown in Fig. 1. Photographs are of a white object on a black ground.)