The surface plasmon resonance (SPR) is an optical method that allows the study of interactions of biological objects with solid surfaces in real time in situ. In the present study the adsorption of ferritin on gold surface was investigated by using a null ellipsometer. The dependence on the optical parameters of adsorbed protein under normal physiological conditions was obtained. A comparison with another method is made. The experiments performed show that ellipsometry is a reliable and correct method for determining of SPR.