We report here the development of an optical biosensor based on the resonant mirror for kinetic analysis of soluble Interleukin-1 receptor I (sIL-1R I) in solution binding to immobilized Interleukin-1alpha (IL-1alpha). IL-1alpha was immobilized through its surface amine groups via amide bonds with the carboxyl groups of the carboxymethyl dextran (CMD) on cuvette surface. The interaction of sIL-1R I and IL-1alpha was monitored in real time. Evaluation of the binding curves allowed the analysis of the binding kinetics. The linear range of sIL-1R I in solution was over a range of 100-1600nM (R=0.9962). Equilibrium dissociation constant (K(D)) was derived by Scatchard plot analysis for sIL-1R I binding to immobilized IL-1alpha. For this assay, the K(D) was 2.6x10(-6)M. The CMD cuvette modified by IL-1alpha was successfully regenerated using 10mM HCl, and the same sensing surface was used repeatedly for the interaction analysis.