The interaction of recombinant human interleukin-6 (IL-6) with the soluble extracellular form of its receptor (sIL-6R) has been characterized by the application of expressions developed for quantitative affinity chromatography to results obtained with a biosensor based on surface plasmon resonance detection. First, the interaction of sIL-6R with IL-6 covalently attached to the biosensor-chip was characterized from the dependence of the surface plasmon resonance response upon the concentration of receptor injected into the biosensor. A binding constant for the interaction between sIL-6R and IL-6 was then determined from the biosensor response observed for mixtures of IL-6 and receptor-a procedure that is shown to provide unequivocal characterization of the competing reaction, irrespective of the model used to describe the biphasic interaction between partitioning receptor and immobilized IL-6. A binding constant of 5 x 10(7) M(-1) has been obtained for the interaction of sIL-6R with two equivalent and independent sites on an essentially dimeric IL-6 preparation produced using the pUC vector system, and also for the interaction of sIL-6R with a monomeric IL-6 preparation that was univalent in its interaction with receptor.