Abstract: A process, means and devices for the quantitative determination, using electromagnetic radiation, of a component of a group including specifically bonding receptors and substances which can be specifically bonded by these receptors are based on a process comprising incubating one of the components, after it is immobilizing on the surface of a material having refractive index different from that of the components, with the other particular component and, optionally, further components, which can be labelled; irradiating the surface with electromagnetic radiation which is polarized parallel to the plane of incidence, the angle of incidence of the radiation being approximately equal to the angle .PHI. at which the intensity of the radiation reflected from the uncoated or precoated surface reaches a minimum; and ascertaining the intensity of the reflected radiation as a measure of the concentration to be determined.