Peripheral polyneuropathy associated with recurrent diarrhoea and orthostatic hypotension was observed in two unrelated German kinships and two sporadic cases. Congo red staining and polarization microscopy of biopsy specimens revealed amyloid deposits. Immunohistochemical investigation using the indirect immunoperoxidase staining with antisera to several purified amyloid fibril proteins showed a positive reaction with an antiserum to the prealbumin-related AF-amyloid in the families and one of the sporadic cases and with an antiserum to the immunoglobulin light chain amyloid (A lambda) in the other sporadic case. Therefore, the amyloid of the families and one of the sporadic cases was identified as the prealbumin-related AF amyloid, while the amyloid of the other sporadic case was of immunoglobulin origin. It is concluded that immunohistochemistry with antisera to the different amyloid proteins is useful in the differential diagnosis of amyloid neuropathy.