AbstractA cytoskeleton fraction enriched in intermediate‐sized filaments and resistant to high and low salt buffer containing Triton X‐100 was prepared from dissociated mouse liver cells and mouse liver homogenates. One‐dimensional sodium dodecyl sulfate‐polyacrylamide gel electrophoresis revealed a family of polypeptides in the relative molecular weight (Mr) range from 41,000 to 55,000, with two polypeptides (component A, Mr55,000; component D, Mr48,000) as major constituents. Antibodies to isolated cytokeratin components of lower molecular weights (48,000 and 45,000 to 41,000) decorated, in indirect immunofluorescence microscopy, a distinct cytoplasmic filament meshwork in hepatocytes of various species whereas antibodies to higher molecular weight components were less or even not effective. The antibodies also reacted with cytokeratin filaments present in epithelial cells of various other organs and species, confirming the widespread occurrence of immunologically related cytokeratin filaments in epithelial tissues. The antibodies to higher molecular weight components showed a higher affinity to cytokeratin filaments of epidermis, stratified epithelia, and myoepithelial cells compared to epithelial cells of other organs, including small intestinal crypt epithelium and secretory cells of the submandibular gland. These observations further support the biochemical and immunologic heterogeneity in the cytokeratin family.Mallory bodies (MBs) present in murine and human livers specifically reacted not only with antibodies to lower molecular weight polypeptides, but also even more so with antibodies to higher molecular weight components, indicating (i) the relatedness of MB material to hepatocyte cytokeratin, and (ii) differences of exposed antigenic determinants of cytokeratin(s) during MB development. In polypeptide composition, MB material differed from hepatocyte cytokeratin by polypeptides of molecular weights in the range of 55,000 to 66,000, which is more reminiscent of the pattern of epidermal cytokeratin. The development of MBs was associated with severe derangement of the cytokeratin filament meshwork in the affected hepatocytes, as revealed by immunofluorescence microscopy, indicating that cytoskeletal material progressively coalesced and was incorporated into MBs. The results present the first demonstration of the hepatocyte cytokeratin meshwork and an example of a pathologic change of this structure. The observations suggest that liver disorders associated with MB development can be regarded as cytoskeleton d