Specimens of normal peripheral nerve and a series of peripheral nerve lesions have been immunostained with three different anti-epithelial membrane antigen (EMA) monoclonal antibodies. Sites of EMA immunoreactivity have been confirmed within perineurial cells of peripheral nerve, noted within the capsule of Schwannomas and palisaded encapsulated neuromas, and also detected with traumatic neuromas and plexiform neurofibromas. No expression was detected within simple neurofibromas, diffuse neurofibromas or within malignant Schwannomas. These sites of EMA expression concur with the suggested involvement of perineurial cells in the formation of the particular lesions. The relationship between EMA expression by the perineurium and the piaarachnoid membrane is discussed.