NCA was purified from normal human lung (L-NCA) and from liver metastases of colon adenocarcinoma (T-NCA), L-NCA and T-NCA had different carbohydrate compositions, but showed an immunological identity in double immunodiffusion with the use of anti-CEA and anti-L-NCA sera. In SDS-polyacrylamide gel electrophoresis L-NCA and T-NCA showed similar molecular weights of about 110,000 and 120,000, respectively. However, if the samples were heated for 3 min at 100 degrees C in the presence of 1% SDS before electrophoresis, their apparent mol. wt decreased to about 50,000, suggesting the dissociation of NCA molecules into subunits. The dissociation of NCA was independent of the presence of mercaptoethanol and did not destroy the ability of NCA to precipitate with anti-CEA and anti-NCA sera. Besides NCA (and CEA in tumor tissue), the presence of a lower molecular weight cross-reacting antigen was observed in lung and tumour. Double immunodiffusion showed that this additional antigen was not a dissociated form of NCA.