The measurements of the amounts of various hormones in the body is one of the most important subjects in the field of endocrinology. The result obtained is not only helpful for the basic studies, such as the function studies of each organ or their interrelationship, but also valuable for routine clinical diagnosis. For the most of peptide hormones or protein hormones, a chemical measurement is very difficult. Biological methods are mostly utilized for this purpose but are not always satisfactory. The use of labeled hormone in combination with its antiserum led to the highly specific and sensitive measurement of the hormone in human plasma. This method is based essentially on the principle of isotope dilution method and is called radioimmunoassay. From the nuclear medical aspect, this is now one of the major fields of in vitro assay with radioisotope. With this method, less than 1 micro unit of insulin per ml of serum can be detected. In this lecture, I would like to talk about the principle and the method of the radioimmunoassay.