The specificity of endothelial cell leukocyte adhesion molecule-1, ELAM-1, for binding to a panel of carbohydrate structures was determined by a sensitive cell binding assay with immobilized synthetic glycoconjugates. ELAM-1 cDNA transfectants were found to bind Sialyl Lea (sialylated lacto-N-fucopentaose II) or sialylated Lewis a antigen (NeuAc alpha 2-3Gal beta 1-3(Fuc alpha 1-4)GlcNAc), as well as or slightly better than Sialyl Lex (sialylated lacto-N-fucopentaose III) or sialylated Lewis X antigen (NeuAc alpha 2-3 Gal beta 1-4(Fuc alpha 1-3)GlcNAc). A monoclonal antibody, HECA-452, which has been identified recently as recognizing ELAM-1 ligands in addition to those containing Sialyl Lex, was also found to bind both Sialyl Lex and Sialyl Lea. Hard sphere exo-anomeric (HSEA) calculations were performed on these two hexasaccharides. The conformations indicate that Sialyl Lea and Sialyl Lex show a high degree of similarity in both the nonreducing and reducing termini. As Lea and Lex show much weaker reactivity, the determinants recognized by ELAM-1 and HECA-452 probably involve neuraminic acid and fucose residues which on one face of both Sialyl Lex and Sialyl Lea can be similarly positioned. The finding that Sialyl Lea is a potent ligand for ELAM-1 is important, as circulating Sialyl Lea and Sialyl Lex containing mucins which are elevated in the serum of many cancer patients may block leukocyte interactions with ELAM-1 and may contribute to the pathological immunodepression observed in these patients.