1. A renal adenylate cyclase system was assessed for its suitability in the performance of parathyroid hormone (PTH) bioassays. Membrane preparations from 1 week old chicks were found to be more sensitive to PTH than material from humans, dogs or rats.

2. Because of the presence of non-specific inhibitors and stimulators of adenylate cyclase in human serum, each serum sample was assayed in the presence and in the absence of a specific PTH inhibitor and the difference was used to calculate PTH activity.

3. Calcium, an inhibitor of adenylate cyclase, was either removed from serum samples by pretreatment with Chelex resin or chelated during assay by means of EGTA.

4. Human and bovine PTH (1-34) stimulated adenylate cyclase in this system to the same extent. The lower limit of detectability was 19.5 pg/ml (final concentration). The intra-assay coefficient of variation at a final concentration of 45 pg/ml was 18%. The index of precision was 0.08 ± 0.04 (n = 7).

5. When synthetic human PTH (1-34) was infused into three normal volunteers, the mean biological half-life of this material was found to be 3.2 min.

6. PTH-like bioactivity could be routinely detected in sera from normal rats, while such activity was significantly decreased in rats subjected to thyroparathyroidectomy.

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