A monoclonal antibody derived from a mouse immunized with bovine epidermal prekeratin has been characterized by its binding to cytoskeletal polypeptides separated by one‐ or two‐dimensional gel electrophoresis and by immunofluorescence microscopy. This antibody (KG 8.13) binds to a determinant present in a large number of human cytokeratin polypeptides, notably some polypeptides (Nos. 1, 5, 6, 7, and 8) of the ‘basic cytokeratin subfamily’ defined by peptide mapping, as well as a few acidic cytokeratins such as the epidermis‐specific cytokeratins Nos. 10 and 11 and the more widespread cytokeratin No. 18. This antibody reacts specifically with a wide variety of epithelial tissues and cultured epithelial cells, in agreement with previous findings that at least one polypeptide of the basic cytokeratin subfamily is present in all normal and neoplastic epithelial cells so far examined. The antibody also reacts with corresponding cytokeratin polypeptides in a broad range of species including man, cow, chick, and amphibia but shows only limited reactivity with only a few rodent cytokeratins. The value of this broad‐range monoclonal antibody, which apparently recognizes a stable cytokeratin determinant ubiquitous in human epithelia, for the immunohistochemical identification of epithelia and carcinomas is discussed.

The organization of intermediate-sized filaments (IF) of the cytokeratin type was studied in cultures of PtK2 cells in which typical IF structures are maintained during mitosis, using a monoclonal antibody (KG 8.13). This antibody reacts, in immunoblotting experiments, with the larger of the two major cytokeratin polypeptides present in these cells but, using standard immunofluorescence microscopy procedures, does not react with the cytokeratin filaments abundant in interphase cells, in striking contrast to various antisera and other monoclonal cytokeratin antibodies. In the same cell cultures, however, the antibody does react with cytokeratin filaments of mitotic and early postmitotic cells. The specific reaction with cytokeratin filaments of mitotic cells only is due to the exposure of the specific immunologic determinant in mitosis and its masking in interphase cells. Treatment of interphase cells with both Triton X-100 as well as with methanol and acetone alters the cytokeratin filaments and allows them to react with this monoclonal antibody. A similar unmasking was noted after treatment with buffer containing 2 M urea or low concentrations of trypsin. We conclude that the organization of cytokeratin, albeit still arranged in typical IF, is altered during mitosis of PtK2 cells.Intermediate-sized filaments (IF) j of vertebrates are cytoplasmic structures that are notoriously stable, both mechanically and chemically, and resist extractions in buffers of a broad range of ionic strengths and pH values (for reviews, see references 1-4). Different types of IF have been distinguished by subunit polypeptide composition and by immunological criteria and their specific expression has been related to routes of cell differentiation (1-3, 5-7). Yet they all share some common principles of morphology and homologies of amino acid sequence (1)(2)(3)(4)(8)(9)(10). In spite of their remarkable stability, IF may undergo redistribution of filaments as well as rearrangements of subunit proteins in living cells. It has been described that arrays of vimentin filaments are reorganized during mitosis (11-15) and drug-induced perinuclear aggregation (11, 16-17) and similar observations have been made for cytokeratin IF in mitosis of some epithelial cells (6,12,18,19). In most cases such re-distributions have been inter- preted as altered distributions of intact IF that do not involve intrafilamentous changes such as disassembly and re-assembly of IF subunits, in agreement with electron microscopic observations of normaMooking IF in all stages of mitosis of various cultured cells of mesenchymal (11, 13, 15) and epithelial (19) origin. By contrast, certain epithelial cells exhibit a drastic, transient change of IF organization during mitosis in which IF are unravelled into different, yet still insoluble and polymeric structures that aggregate into variously sized, spheroidal masses containing cytokeratin (20)(21)(22). These observations suggest that the structural state of at least certain IF is physiologically regulated. In...