The differentiation of the keratinocytes of the human palatine tonsils were studied by means of light and electron microscopy and immunohistochemistry using a polyclonal (K) and two monoclonal antikeratin antibodies (PKK1, PKK2). In the surface epithelium, the basal cells, cuboidal or columnar in shape, undergo progressive terminal differentiation and are transformed into the flattened cells of the upper layers. K reacts with both the basal and spinous layers, while PKK1 and PKK2 mark exclusively the basal layer. In the neck portion of the crypt, cavities containing one or aggregated lymphocytes with amorphous substances are observed in the spinous layer. The cavities are surrounded by elongated cytoplasmic processes of transformed epithelial cells bearing surface microvilli. These transformed epithelial cells display intense PKK1- and PKK2-positive reactions, whereas other conventional polygonal cells in the vicinity remain PKK1- and PKK2- negative as do those in the surface epithelium. In the deep portion of the crypt, where numerous lymphocytes invade the epithelium, the epithelial cells are transformed into star-shaped reticulum cells showing PKK1- and PKK2-positive reactions. The extended and branched cytoplasmic processes interconnect with one another constituting a complex network of reticulum cells, the well known reticulation of the crypt epithelium. Ten-nm filaments are usually oriented parallel to the longitudinal axis of transformed epithelial cells. Our observations suggest that the cell-shape transformation, i. e., from conventional polygonal epithelial cells into epithelial reticulum cells, occurs when the epithelial cells are in close contact with the infiltrating lymphocytes, and that this transformation is accompanied by a change in keratin phenotype.