In ground sections from 100 unerupted human premolars examined by reflected and polarized light, the appearances of the outer enamel indicated a porous structure which was confirmed by SEM of three-dimensional methacrylate replicas. The width of the porous outer enamel was assessed from drawings of projected transparencies of buccolingual sections photographed by reflected light of 41 sections from 16 upper premolars and 30 sections from 12 lower premolars, as well as 12 sections from 5 partly erupted premolars. The findings indicate that the outer enamel of premolar teeth is porous at the time of eruption.