An antigen-antibody complex of horseradish peroxidase, peroxidase-antiperoxidase (PAP), was found to selectively bind to Mallory bodies (MBs). Specific PAP binding to MBs was consistently demonstrated in liver sections from 14 patients with alcoholic cirrhosis and from one patient with hepatoma. Mallory bodies in isolated fractions also bound PAP. No structures stained with PAP in six control sections. The PAP-binding method is useful in the identification of MBs in situ and in isolated fractions. The nature of the affinity of MBs for PAP is not known, but it is postulated that nonspecific protein-immunoglobulin binding is involved.