The fine structural morphology of the filiform papillae of the human tongue is investigated by transmission and scanning electron microscopy. Separation of the epithelium from the connective tissue by treatment with trypsin allows the demonstration of the three-dimensional structure of the primary and secondary connective tissue papillae and of the papillary epithelium. The distal surface of each primary connective tissue papilla is studded with secondary connective tissue papillae. Their lamellated surface structure corresponds with a complex system of cytoplasmic processes of the basal epithelial cells. The complicated surface architecture of the epithelial-connective tissue junction may suggest a close structural and functional relationship between the two types of tissues. There are two distinct cell populations forming the epithelial papilla, which differ in ultrastructural morphology from the basal cell layer to the surface of the epithelium. A dual pattern of keratinization, phylogenetically analogous to the hard and soft keratinization in rodent tongues, is found also in filiform papillai of the human tongue. The final product of epithelial differentiation is a type of keratin different from that in epidermal keratinocytes and in other regions of the oral cavity. The variability in size, shape and association with other organelles suggests the heterogenous nature of lingual keratohyaline granules. The predominance of Odland bodies of lamellated or of spoke type varies with the degree of keratinization of different regions of the papillary epithelium.