Blood group determinants have been found in five carcinoembryonic antigen (CEA) preparations with a lectin and antibody-binding assay using polyethylene glycol 6000 to separate free from bound radiolabeled antigen. The assay described gives excellent sensitivity for the binding or inhibition of binding of various lectins or antibodies to CEA. One of the CEA preparations investigated has an A1 determinant, another has a B determinant, and all have H, Lea, Leb, and MN blood group determinants. In addition, all of the preparations tested bound concanavalin A. These findings are consistent with the idea that incomplete or unexpected glycosylation patterns occur in glycoproteins produced by tumor cells. Since antibodies directed against blood group substances cross react with carbohydrate determinants on CEA, clinical determinations of CEA or anti-CEA levels in serum may be adversely affected.