Interactions between protein toxins and carbohydrate receptors are often semi-selective processes and the kinetic parameters that define the binding of a receptor to different toxins may vary with each interaction. In this study, we have determined the affinity constants for binding of cholera toxin (CT) to immobilized sialic acid and to anti-CT antibody (as a simultaneous reference) by measuring real-time binding processes using an array biosensor. N-Acetylneuraminic acid (Neu5Ac), a member of the sialic acid family, was covalently immobilized onto maleimide-activated planar waveguides via a thiol-terminated linker attached to the anomeric carbon of the sugar. Control antibodies were immobilized using two different approaches: covalent attachment onto maleimide-activated slides via the thiol on cysteine residues and non-covalent attachment using a biotin-NeutrAvidin linkage. Cy5-labeled CT was flowed over the immobilized receptors and the fluorescent intensity of the bound CT-receptor complex was recorded as a function of time. The association constants for CT binding to covalently attached Neu5Ac, to covalently attached anti-CT monoclonal antibody, and to antibody tethered by biotin-NeutrAvidin interactions were determined to be 1.3 x 10(8), 2.1 x 10(8) and 5.7 x 10(8)M(-1), respectively.