Mallory bodies (MBs) induced in hepatocytes by long term feeding of mice with griseofulvin were isolated, purified, and examined by electron microscopy in ultrathin sections and negatively stained preparations. The major structural component of the MBs was randomly oriented, unbranched rods of filaments; these were usually 175 to 250 nm. long and 14 to 20 nm. thick and covered by a dense fimbriate coat of laterally projecting 1.5- to 3-nm. thick threads. Such lateral threads could extend for more than 20 nm. and seemed to be involved in the interconnection of adjacent filaments and their association and aggregation into MBs. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the purified filament material showed six major polypeptide bands with apparent molecular weights ranging from 48,000 to 66,000. When the portion of the MB filament material that was soluble in solutions containing 8 M urea was allowed to reaggregate upon removal of the urea, an enrichment of one of the polypeptide components (approximate molecular weight, 64,000) was observed. When frozen sections of liver tissue MBs were subjected to indirect immunofluorescence microscopy, they were specifically revealed by guinea pig antibodies directed against purified bovine prekeratin. No significant accumulation of MBs was observed with a series of other antisera, including those containing antibodies against tubulin, actin, and vimentin, the major protein of the intermediate sized filaments predominant in mesenchymal cells. The observations suggest that MBs in livers of griseofulvin-treated mice, and probably also of human alcoholic hepatitis, contain large amounts of prekeratin-like polypeptides which are assembled into a special form of fimbriated rods of 14- to 20-nm. filaments. These filaments are morphologically different from other forms of intermediate sized and thick filaments, including the prekeratin-containing 6- to 11-nm. tonofilament-like filaments present in various epithelial cells.