SUMMARY: An immunoradiometric assay for parathyroid hormone has been developed. Antisera to bovine parathyroid hormone (BPTH) were screened for their ability to bind BPTH and human parathyroid hormone (HPTH). A BPTH-immunoadsorbent was used to extract antibodies from an antiserum which did not discriminate between BPTH and HPTH in a standard radioimmunoassay. These antibodies were labelled with 125I for use in the immunoradiometric assay. With this system as little as 5 pg BPTH and 8 pg HPTH could be detected. The serum concentration of BPTH was shown to rise in a cow rendered hypocalcaemic by an infusion of EDTA. Sera from patients with hyperparathyroidism contained high concentrations of hormone. These sera were found to dilute-out parallel to calibration curves obtained using HPTH extracted from parathyroid adenomata. The advantages of this method over the standard radioimmunoassay are discussed.