Three immunogenic parts have so far been identified in the carcinoembryonic antigen (CEA) molecule. These are: determinants cross-reactint with the normal fecal antigen (NFA) (NFA determinant); determinants cross-reacting antigen (NCA) (NCA determinant); and determinants which appear to be more cancer-specific (cancer determinant). The chemical nature of these parts of the CEA molecule was investigated by digestion with proteolytic enzymes together with anti-CEA preparations with which these three immunogenic parts of CEA molecule could be identified. The CEA digest obtained with pepsin did not react in immunodiffusion and radioimmunoassay, indicating that pepsin completely destroyed all the antigenic parts. Digestion by pronase E destroyed only the cancer determinant and liberated two antigenic fragments corresponding to the NFA determinant and the NCA determinant, respectively. These results suggest that the cancer determinant may reside in a protein or a peptide part of the molecule. The chemical nature of the NFA and NCA determinant remains to be clarified.

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