The Proteon XPR36 exploits the optical phenomenon of surface plasmon resonance (SPR) to provide real time monitoring of interactions between molecules. A molecule (the ligand) is immobilized on a gold surface using any of a variety of available chemistries. The interaction of light with the gold surface generates a surface plasmon whose properties depend on the amount of material at the surface. When a solution containing other molecules (analytes) whether pure or in a complex mixture (e.g. a cell homegenate) flows over the ligand surface any binding interactions increase the mass of material at the surface altering the properties of the SPR signal. The changes in signal are monitored in real time, permitting one to observe association rates, equilibrium binding levels and dissociation rates from the surface. Because the method involves direct measurement of changes in mass at the surface, neither ligand nor analyte need be labeled with radioactivity or spectral probes so that aside from the immobilization, minimal alterations to the molecules involved are required.