The calcitonin genes encode a small family of peptides: the circulating hormone calcitonin; its flanking peptide, katacalcin; and a third novel peptide, calcitonin gene-related peptide (CGRP). CGRP is a potent vasodilator and a major circulating product from the calcitonin genes; it may be a physiologically important regulator of blood flow in humans. High concentrations of circulating CGRP are found in medullary thyroid carcinoma. We report the development and validation of a highly sensitive (detection limit 500 amol per tube) radioimmunoassay of CGRP involving a high-affinity antibody directed against the carboxyl terminus of the molecule and a highly pure tracer. The assay is precise, robust, and reproducible, and is therefore a potentially useful analytical method for studying the normal and abnormal physiology of this peptide.