A radioimmunoassay for the measurement of immunoreactive parathyroid hormone (PTH) in human serum is described. The assay is based on the ability of human parathyroid hormone (h-PTH) to compete with 125I-labelled bovine parathyroid hormone (b-PTH) for binding to a guinea-pig antiserum directed against b-PTH. The linear part of the standard curve was parallel with dose response curves for anti-b-PTH serum reacting with dilutions of sera from patients with primary hyperparathyroidism and from h-PTH purified from human parathyroid adenomas, indicating that levels of immunoreactive PTH could be expressed as b-PTH equivalents. The range in 62 healthy blood donors was 1.1-2.5 ng b-PTH Eg./ml. The reproducibility was satisfactory, and the sensitivity permitted the measurement of PTH concentrations down to 0.8 ng b-PTH Eg./ml. No crossreaction with h-CT, h-STH or h-ACTH was observed. The clinical value of the assay has been considered in a number of patients with various disorders of calcium metabolism, diagnosed and treated conventionally. About 80 per cent of patients with primary hyperparathyroidism had elevated PTH levels on one or more occasions before surgery. In patients with chronic renal failure of other aetiology than primary hyperparathyroidism the levels were usually far higher. Patients with primary hyperparathyroidism and increased S-creatinine had higher PTH levels than those with normal S-creatinine. After parathyroidectomy all previously increased PTH levels became normal or low. High PTH concentrations were found in 3 patients with normocalcaemic hyperparathyroidism who at operation were shown to have parathyroid adenomas. However, in normocalcaemic patients there were also some falsely elevated PTH values which limit the diagnostic value of the assay in this group of patients. Low PTH values were observed in patients with hypercalcaemia due to malignant disorders, indicating that PTH determination may be of some value in the diagnosis of patients with hypercalcaemia of unknown origin.