The invention concerns a device and a method for carrying out fluorescence immunoassays, wherein from at least one light source light is directed onto a surface at one end of a light waveguide and with the light coupled into the light waveguide by evanescent field excitation at the surface of the light waveguide fluorescence of at least one labelling substance bound to a chemical or biochemical partner of a general receptor-ligand system is excited. The solution according to the invention is here to provide a possible way of carrying out fluorescence immunoassays with high accuracy of measurement at low cost and within a short time. To achieve this object, the fluorescent light is decoupled from the light waveguide and directed via an optical system onto an optical detector with which the intensity of the fluorescent light is measured. Here the light waveguide is held in a measuring chamber formed in a piston of a piston and cylinder unit, and the measuring chamber is connected to the interior of the cylinder by an inlet formed in the piston.