The radioimmunoassay technique, first developed for the determination of hormones, has been applied to many substances of biologic interest by clinical and research laboratories around the world. It has had an enormous effect in medicine and biology as a diagnostic tool, a guide to therapy, and a probe for the fine structure of biologic systems. For instance, the assays of insulin, gastrin, secretin, prolactin, and certain tissue-specific enzymes have been invaluable in patient care. Further refinements of current methods, as well as the emergence of new immunoassay techniques, are expected to enhance precision, specificity, reliability, and convenience of the radioimmunoassay in both clinical and research laboratories.