Material reacting like carcinoembryonic antigen (CEA) in the radioimmunoassay has been extracted from rheumatoid synovial membranes. This CEA activity has been compared to that found in hepatic metastases from colorectal tumours and in normal adult lung. The antigen in the rheumatoid synovium has been shown to be more sensitive to perchloric acid and to isolate with a lower weight than that derived from the tumour and lung. Immunodiffusion studies with anti-CEA indicate that the CEA-like determinants in the rheumatoid synovium have partial identity with tumour CEA and that a significant proportion of them are associated with large molecular weight material. Production of an antiserum to these CEA-like components in the rheumatoid synovium should enable further identification of their relationship to tumour CEA and might allow a better judgement of whether or not they represent the expression of neoantigens in the disease.