Keratin extracts from the epidermis of adult mouse ears, footpads, and tail contain large amounts of a 70-kilodalton (kDa) protein which has not been detected in any other body site of the adult mouse or in the epidermis of neonatal mice. Two-dimensional immunoblotting using an antiserum which recognizes both type-I and type-II murine keratins revealed that the 70-kDa protein is indeed a keratin belonging to the type-II subfamily. Its postnatal induction occurs during the first 2 weeks after birth, being first observed in tail epidermis, then in footpad epidermis, and only rather late in ear epidermis. Although in vitro translation experiments with polyA+-RNA from adult tail and footpad epidermis consistently failed to reveal the 70-kDa protein among the translation products, we obtained evidence using a specific cDNA clone that, in vivo, the protein is encoded by a discrete mRNA. This clone, termed pke70, was isolated from a cDNA library of footpad epidermal mRNA. Homology comparisons with a variety of known keratin cDNAs indicated that pke70 contains sequence information for a type-II keratin that is substantially larger than the mouse 67-kDa keratin protein. Northern-blot analysis with a specific 3'-fragment of pke70 demonstrated a single 2.8 +/- 0.1 kb mRNA species exclusively in adult ear, footpad, and tail epidermis. In situ hybridization with the same fragment revealed the presence of the pke70-hybridizing mRNA in both basal and suprabasal cells of ear and footpad epidermis as well as in the orthokeratinizing parts of the tail epidermis; however in the epidermis covering the balls of the feet, labeling was restricted to suprabasal cells at the base of these nodular elevations. Continuous treatment of adult tail or ear epidermis with hyperplasiogenic agents, e.g., vitamin A acid and the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA), leads to a gradual disappearance of the 70-kDa protein. We obtained evidence using in situ hybridization that the loss of the 70-kDa keratin is preceded by a specific suppression of the transcription of its putative mRNA in basal cells, whereas initially suprabasal cells are apparently still able to complete their original commitment. The particular properties of the 70-kDa keratin protein, i.e., its topological restriction, its postnatal and time-dependent acquisition, and its pronounced sensitivity to hyperplasiogenic stimuli, make this keratin subunit an especially suitable candidate for studies concerning the regulation of keratin expression and morphogenesis in general, as well as for studies of the factors that control its expression so specifically.