The technic of radioimmunoassay developed by Berson and Yalow 1 is based on the competitive inhibition of binding of radioactively labeled hormone to specific antibody. There must be available pure hormone antigen for labeling with radioisotope, usually iodine 131 or iodine 125. The ratio of antibody-bound hormone labeled with 131I (B) to free hormone labeled with 131I (F) is a function of hormone concentration. By measuring the ratio B/F after incubation with dilute mixtures of antibody and labeled hormone, a sensitive immunoassay can be obtained which can be applied to detection of hormone in peripheral plasma. A standard curve for the assay of parathyroid hormone is illustrated in Fig 1. The graphical analysis shown provides more than quantitation alone; by analyzing the shape of the curves one can derive important qualitative information regarding the nature of an unknown hormone sample. For example, a hormonal preparation immunological identical to