Recent advances in carcinoembryonic antigen (CEA) research were reviewed. The primary structure of CEA peptide, mode of membrane anchoring of CEA, heterogeneity of carbohydrate chains, which seemed to be a main cause of the variations in CEA titers of patient's sera, and a newly established monoclonal radioimmunoassay system that revealed very homogeneous reactions with all CEA preparations and improved results on clinical applications were described. Furthermore, a recent finding that CEA is produced by normal colon mucosa as actively as by cancerous tissue was discussed.