To investigate the possibility that Mallory bodies (MBs) are composed of intermediate filaments (IFs), the electrophoretic patterns of MB proteins from human liver were compared with the IF protein extracted from chicken gizzard smooth muscle and extracts of mouse liver which contained MBs. The human MB had a protein component which had a similar mobility as that of the IF protein extracted from smooth muscle. A similar protein band was found in the extracts of mouse liver which contained MBs. No protein band corresponding to actin protein was found in human MB protein. The IF-like protein was also present in control mouse liver extract. The estimated molecular weight of the IF protein was 54,000. The evidence suggests that MBs are, in part, composed of IF protein. The IF protein is a normal constituent of liver and smooth muscle.