We present the results obtained using a 1-84 human parathyroid hormone (PTH) assay in patients with abnormal calcium metabolism. A normal range was established in a series of healthy volunteers. Patients with surgically proven hyperparathyroidism (HPT) showed increased PTH levels which were separate from the normal range. Patients with probable hyperparathyroidism and milder hypercalcaemia showed a raised median PTH level but the range overlapped with normal. Patients with hypercalcaemia of malignancy showed reduced PTH levels and these patients were readily differentiated from those with probable and proven HPT by the use of the assay. In patients with chronic renal failure PTH values ranged from normal to high, the PTH concentration was found to be correlated with plasma alkaline phosphatase, but not with plasma creatinine.