Abstract The immunochemical relationship between membrane-associated CEA (CEA-M) and isolated conventional whole CEA and the species CEA-S was investigated. These glycoproteins gave precipitin reactions of apparent identity by using antisera to CEA-M and CEA-S; however, the binding of each of these glycoproteins relative to one another by anti-tumor glycoprotein, anti-CEA-S, and anti-CEA-M anti-sera differed to a significant degree, indicative of some degree of nonidentity. Competitive inhibition radioimmunoassay with a 125I CEA-S ligand and a broadly reactive anti-CEA-S antiserum demonstrated that all three glycoproteins possessed in common antigenic determinants of the CEA group of glycoproteins but at apparent different concentration. These appeared to reflect the presence of CEA group-specific determinants. On the other hand, when 125I CEA-M and an anti-CEA-M antiserum were used in competitive inhibition assays, both CEA and CEA-S were antigenically deficient relative to CEA-M. After absorption of the anti-CEA-M with CEA, CEA-M produced complete inhibition, SDS-unfolded CEA produced 60% inhibition and CEA was unreactive. This antiserum was further absorbed with SDS-unfolded CEA and used in assay with 125I CEA-M. Although CEA-M remained reactive, CEA or CEA subjected to denaturants such as SDS, 2-mercaptoethanol, and SDS with 2-mercaptoethanol did not inhibit this assay system, indicating the absence of the remaining determinants present on CEA-M. Based on these observations we have proposed the existence not only of CEA group-specific determinants but also the existence of CEA cryptic determinants that may be generated from soluble forms of CEA and CEA-S by unfolding and exposure of cryptic sites. In addition, the existence of CEA species-specific determinants, which appear to be unique to a particular species of CEA, have been demonstrated. In a CEA-M-specific radioimmunoassay system with CEA-M absorbed antiserum the CEA-M species-specific determinants can be directly demonstrated in micro-some-rich homogenates of colonic adenocarcinomas. Thus, molecules bearing CEA-M species-specific determinants are produced by cells and are not an artifact of isolation of CEA-M. Demonstration of this third class of determinants on CEA-M in this study establishes that it is an independent species of CEA.