Radioimmunoassay is an analytic method which combines the sensitivity of radioactive measurement and the specificity of the antigen-antibody reaction Substances down to a concentration of some picograms per ml serum (or biological material) can be measured in the presence of a millionfold excess of otherwise interfering substances. The method is easy to carry out (test tube chemistry). The main field of application at the moment is in endocrinology; further possibilities of application are in pharmaceutical research, environmental protection, forensic medicine, and for general analytic purposes. Radioactive sources are used only in vitro in the nanocurie range, i.e. radiation exposure is negligible.