The efficiency of three parathyroid hormone (PTH) assays in the diagnosis of primary hyperparathyroidism has been investigated. Two assays used commercial reagents and measured the intact hormone and middle region of parathyroid hormone and the third assay employed the antiserum AS 211/32. Although the intact and mid-molecule assays showed a greater increase in PTH above normal than the laboratory PTH assay, patients with proven primary hyperparathyroidism were not distinguished from healthy subjects in every case. The intact and in house assays gave elevated values in seven out of nine patients with proven hyperparathyroidism and the mid-molecule assay in eight out of nine assays. The intact PTH and in-house assays were more successful than the mid-molecule method in separating patients with primary hyperparathyroidism from those with hypercalcaemia due to malignancy.