Carcinoembryonic antigen (CEA) has been reported in benign and malignant epithelium of the gall bladder, although cross-reactivity with CEA-related antigens cannot be excluded. We have investigated the immunoreactivity of three monoclonal and two polyclonal antisera to CEA in benign and malignant epithelium of the gall bladder, extrahepatic bile ducts, and ampulla of Vater. The results varied with the antisera, due to cross-reactivity with CEA-related antigens and according to the epitope specificities. Little CEA was found in benign epithelium using monoclonal antisera or following absorption of a polyclonal antiserum from Dako with human liver powder. The latter was the most reliable at detecting malignant epithelium of the gall bladder and extrahepatic bile ducts. All antisera showed equal immunoreactivity in tumours of the ampulla of Vater. The differences seen in the immunoreactivities between tumours of the upper and lower extrahepatic biliary tract probably indicate a variation in the occurrence of CEA epitopes.