ABSTRACT— Mallory bodies induced by long-term griseofulvin feeding in mouse liver were isolated and analyzed by one- and two-dimensional gel electrophoresis and reaction of the separated polypeptides with cytokeratin antibodies using the blotting technique. Comparison with normal intermediate filament cytoskeletons from mouse hepatocytes revealed that Mallory bodies contain two polypeptides: Component II (Mr: 55,000; apparent isoelectric point values: 6.45, 6.1, 5.9) and component III (Mr: 48,000; apparent isoelectric point values: 5.7, 5.5, 5.43, 5.38, 5.2) which appear to be similar, if not identical, to liver cytokeratins A and D, respectively. By contrast, component I of Mallory bodies (Mr: 65,000; apparent isoelectric point values: 5.4, 5.38, 5.2) was not found in appreciable amounts in normal hepatocytes. Component II was positive in immunoreaction with antibodies to murine hepatocyte keratins A and D as well as epidermal prekeratin. Component III showed reaction with the antibodies to murine hepatocyte keratins A and D but not with those raised against epidermal prekeratins. By contrast, the unusual component I reacted with antibodies to murine hepatocyte keratin D and to epidermal prekeratins. The results prove that cytokeratin polypeptides are major constituents of Mallory bodies and suggest that the pattern of liver cytokeratin polypeptides is altered during the toxic treatment and/or Mallory body formation.