Abstract: An assay apparatus employing total internal reflection of excitation radiation at the interface between a replaceable optically conductive assay device and a surrounding liquid phase containing the substance being assayed. The assay device is formed of a radiation transmissive, elongated fiber having on the surface thereof a component of a complex formed in an immunological-type specific reaction. A fluorophore that can be excited into fluorescence by the excitation radiation is attached to another component of the complex. The assay device also includes a boss formed of a similarly radiation transmissive material with an index of refraction preferably matched to that of the fiber, one end of the fiber being fixedly coupled to the boss. The boss is dimensioned so that the maximum solid acceptance angle for excitation radiation introduced into the fiber end lies totally within the transmissive medium of said boss.