Real-time biospecific interaction analysis (real time BIA) based on Surface Plasmon Resonance (SPR) has been used to measure the concentration of β2-microglobulin (β2-μ) and Ig E, used as a model system, simultaneously in buffer and plasma. The method relies on specific binding of β2-microglobulin and Ig E to a sensor chip surface where the monoclonal antibodies anti-β2-microglobulin and anti-Ig E were covalently immobilized. The primary binding of the analytes to the surface is followed by the injection, in sequence, of secondary antibodies, each one specific for a different epitope of β2-microglobulin and Ig E, respectively. The binding between the antibodies and the analytes is recorded as an increase in the SPR signal expressed in Resonance Units (RU). The SPR signal is directly related to the amount of β2-microglobulin and Ig E bound to the surface, and depends upon the concentration of the analytes in the sample. The analysis was performed in buffer and serum to show any non-specific binding due to serum proteins. The concentration range 0.35-5.55 nM for both analytes is covered, with good precision and close correlation with an independent standard concentration measurement.