Almost twice as much fatty acid was extracted without demineralization from the mature enamel of erupted teeth treated with 2:1 chloroform-methanol as from the enamel of unerupted teeth (32.64 micrograms/g versus 15.60 micrograms/g). Palmitic C16:0, stearic C18:0, oleic C18:1 omega 9 and linoleic acids C18:2 omega 6 were the chief fatty acids detected by gas-liquid chromatography. The percentage of free fatty acids and diglycerides was larger in unerupted than erupted teeth, whereas that of triglycerides, phosphatidylethanolamine and phosphatidylcholine was larger in erupted teeth. These changes may be due to bacteria and other contaminants (saliva and pellicle). The large amounts of free fatty acids, diglycerides, cholesterol and phospholipids as intrinsic components were probably due to the persistence of membrane remnants entombed during enamel formation, as indicated by the visualization of holes and by the increase in the size and number of focal holes after lipid-solvent interaction with the enamel surface. These defects always occurred in rods and never in inter-rod material. Other minute defects, including empty-widened intercrystal spaces, were also observed inside some rods.