A new calcitonin (CT) immunoradiometric assay, using anti-11-7 and anti-24-32 CT fragment monoclonal antibodies was evaluated and compared to classical RIA. The sensitivity was 2.5 ng/L, the normal basal level (n = 83) was lower than 10 ng/L, the response to pentagastrin stimulation in control subjects was absent in nine and between 10-30 ng/L in nine others. (mean, 15.4). In patients with renal failure the basal level was increased between 10-52 ng/L. In patients with medullary thyroid carcinoma (MTC; n = 28), the basal level was between 189-28,900 ng/L. A pentagastrin test was performed as screening for familial MTC in eight patients with confirmed MTC at subsequent surgery; the calcitonin peak was equal or greater than 38 ng/L. Large differences exist between CT levels measured by RIA and immunoradiometric assay. The latter method provides a greater sensitivity to pentagastrin test and allows a better identification of microcarcinoma in hereditary cases of MTC.