The monoclonal antibody GSLA-2 recognizes the sialyl Lewis(a) (sLe(a)) epitope, which has an increased occurrence on mucin type glycoproteins of patients with colorectal carcinoma. GSLA-2 is therefore used in tumor diagnosis. To advance the understanding of this highly specific molecular recognition reaction, we have analyzed the binding epitope of sLe(a) at atomic resolution using saturation transfer difference NMR. To compare, the binding epitopes of sialyl Lewis(x) (sLe(x)) and of four synthetic derivatives of sLe(a) were explored. Surface plasmon resonance experiments furnished thermodynamic and kinetic data. It is observed that all pyranose rings of sLe(a) are in contact with the protein surface, with the neuramic acid residue receiving the largest fraction of saturation transfer. In contrast, sLe(x) binds very weakly, though specifically to GSLA-2, with a different binding epitope. This study provides a comprehensive picture of the recognition sLe(a) and explains the exquisite specificity of the antibody.