Knowledge of the structure of carcinoembryonic antigen (CEA) is essential if we are to understand the relationship of this molecule to similar, possibly cross-reacting, molecules present in nonmalignant states. Electron microscopy shows that at neutral pH, CEA particles consist of homogeneous, morphologically distinctive, twisted rod-shaped particles, about 9 x 40 nm. The carbohydrate structure of CEA has been studied by periodate oxidation. All the N-acetylneuraminic acid and fucose and a portion of the galactose and mannose were destroyed by the first periodate treatment without altering immunological activity. N-Acetylneuraminic acid was shown to be linked to galactose since its prior removal with neuraminidase led to an equivalent increased destruction of galactose by one treatment with periodate. Significantly, even after 100% of the fucose and N-acetylneuraminic acid, 75% of the galactose, and 50% of the N-acetylglucosamine and mannose were destroyed by serial periodate oxidation (Smith degradation), the remaining portion of the CEA molecule lost no more antigenic activity than did control samples where periodate was omitted. No carbohydrate was lost or destroyed in the control reaction. The carbohydrate portion of CEA has also been studied by methylation analysis using gas chromatography-mass spectrometry. This analysis indicated that the N-acetylneuraminic acid is substituted on position 3 of galactose and that a large proportion of the fucose residues must be linked to N-acetylglucosamine. These methylation studies also indicated that three-fourths of all mannose residues are branching. The nature of the carbohydrate chains, their number, and their placement are being investigated. © 1976, American Association for Cancer Research. All rights reserved.