Radioimmunoassay is an analytical method combining the sensitivity of radioactivity measurements and the specificity of the antigen-antibody-reaction. Thus, substances can be measured in concentrations as low as picograms per milliliter serum besides a millionfold excess of otherwise disturbing material (for example in serum). The method is simple to perform and is at present mainly used in the field of endocrinology. Further areas of possible application are in the diagnosis of infectious disease, drug research, environmental protection, forensic medicine as well as general analytics. Quantities of radioactivity, exclusively used in vitro, are in the nano-Curie range. Therefore the radiation dose is negligible.