The epithelial lining of lateral cervical cysts (LCCs) was analyzed for keratin polypeptide composition by means of high resolution gel electrophoresis and immunoblotting using polyclonal rabbit antikeratin antisera of defined specificity. The keratin phenotype expressed in branchial mass epithelia was found to be homologous to the profiles obtained for the squamous epithelium of corresponding palatine tonsils, but was clearly different from related polypeptide complements of both epidermis and simple (columnar) epithelium. The presence of particular keratin members (pairs 5/14 and 4/13) strongly indicates that branchial mass inner lining derives from keratinocytes that are programmed to form a stratified squamous epithelium and reveal, at least biochemically, an upper digestive tract or esophageal type of differentiation. On the basis of these data and the recent finding that a neck lymph node is involved as a target tissue in LCC formation, hypotheses concerning branchial mass histogenesis in general appear to be highly unsettled. We propose an alternative model that may explain the conflicting clinical, anatomic, and morphologic findings associated with LCC disease.