1. Monolayer cultures of human parathyroid cells were established for a study of the nature of secreted human parathyroid hormone (HPTH). 2. [14C]Leucine and 75Se-labelled methionine were used as amino acid precursors for the biosynthesis of HPTH. 3. Immunoreactive labelled HPTH in the medium was extracted by affinity chromatography. 4. In cultures from glands, six adenomatous and one hyperplastic, radioactive HPTH in the media was indistinguishable in size and charge from highly purified bovine parathyroid hormone (PTH). 5. In two cultures from adenomata, media contained an immunoreactive basic peptide resembling ‘pro-PTH’ in its electrophoretic behaviour. The freeze-dried cells contained the same basic peptide.