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Crystallographic structures and thermodynamic binding parameters are compared for three structural classes of streptavidin ligand including d-biotin, 2-[(4'-hydroxyphenyl)-azo] benzoate and the peptide NH2-Phe-Ser-His-Pro-Gln-Asn-Thr-COOH. Descriptions of these structural and thermodynamic observations emphasize the diversity of potential strategies for improving ligand affinity.