N THE BASIS of clinical location and gross appearance, two types of white opaque enamel may be distinguished: (1) white opaque areas of enamel on the approximal surface of young teeth, commonly referred to as white spots or early various lesions. These are regarded as the result of acid action on enamel, (2) white enamel areas, diffuse in nature, and present as large irregularly distributed bands in enamel, commonly referred to as hypocalcified developmentally defective areas.As a result of histologic observations and the increased stainability of these areas, the belief has been expressed that this altered enamel has a higher organic content than sound enamel. Hardwick and Manley,' and Hodson2 claim that the increase in organic material of such opaque areas is due to the adsorption of material from the saliva. Biochemical studies of white opaque enamel by Coolidge,3 however, revealed no significant difference in the nitrogen content of sound and altered enamel. Stack4 on the other hand, showed that chalky enamel contains 3 to 4 times as much organic material as the adjacent sound enamel. There have apparently been no attempts to determine the density of such areas.The present investigation was undertaken to characterize the two types of opaque enamel as to histologic structure, density, and nitrogen content and to compare these findings with corresponding data on sound enamel. MATERIAL AND METHODSThe procedure for selection of teeth and recording of their history, the fixation and preparation of ground sections, and the special technics (collection of enamel samples; density and nitrogen determinations) used for studying the altered enamel areas have been reported in previous publications (Bhussry5' 6). It should be mentioned, however, that on the basis of clinical

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