A new simple bioassay method for parathyroid hormone (PTH) has been developed. It is based on the rapid and sensitive hypercalcemic response of immature birds. Japanese quail, aged 2–3 weeks and weighing 35–55 g, are injected with standard or unknown doses of PTH (1, 3 and 9 U.S.P. units/ml) using at least 6 birds per group. Preparations are dissolved in 51 mM CaCl2 and injected into a wing vein (0.1 ml/10 g bw). The birds are bled by cardiac puncture 60 min after injection and the plasma calcium determined. Sixty quail can be injected and bled within a period of 2 hr. Parathyroid Injection, U.S.P. (Lilly), is used as a standard. An average index of precision, λ, of 0.20 has been obtained in six assays under these conditions. Purified PTH gives a dose—response slope which is parallel with that of the standard preparation over the dose range tested (3–12 U.S.P. units/ml). The assay is relatively specific; no significant hypercalcemic response was seen with 10 units/ml of inactivated PTH, 100 MRC m units/ml calcitonin, 40 m units/ml vasopressin, 1 μg/ml aldosterone, 16 Mg/ml glucagon, 5 μg/ml P as NaH2PO4, 10 mg/ ml dibutyryl 3',5'–cyclic AMP, or 0.01M cysteine HC1. Preliminary data suggest that 5–6–day—old chickens can also be used as the assay animal. (Endocrinology92: 463, 1973)