To identify Mallory body (MB) constituents, monoclonal antibodies to murine MBs induced by long-term griseofulvin (GF) feeding were produced. One of these, antibody MM 120-1, specifically reacted in immunofluorescence microscopy with MBs in all developmental stages but not with other cell structures of human and mouse liver and other organs. The MM 120-1 antigen was present in murine MBs induced by griseofulvin or 3,5-diethoxycarbonyl-1,4-dihydrocollidine feeding and also in human MBs in livers of patients with alcoholic hepatitis. In immunoblots, the MM 120-1 antigen was detectable in the high molecular weight fraction of MB preparations, most of which remained in the well and at the interphase between stacking and resolving gel. No reactivity with cytokeratin polypeptides of different conformational states (i.e., isolated cytoker atin components A and D, heterotetramers A2D2, reconstituted intermediate filaments) was found. It is concluded that the antibody MM 120-1 is a highly specific and sensitive marker for murine and human MBs recognizing a high molecular weight nonkeratin component. This component could play a central role in the pathogenesis of MBs.