A new method for the bioassay of parathyroid hormone (PTH) has been developed in the chick which is specific, sensitive and precise (mean λ 0.14) and has the convenience of not requiring parathyroidectomy. PTH in a special vehicle is injected iv to 7-14-day-old chicks, which are anesthetized and bled 60 min later for analysis of plasma calcium. The speed of this procedure allows a 3 + 3 assay design (30 birds in groups of 5) to be completed in two hr. The vehicle contains enough calcium chloride to give a dose of 20 nmoles per bird. Although this small quantity leaves the circulation rapidly and is undetectable at the time of bleeding, it doubles the magnitude of responses to PTH, which are linearly related to log dose between 1 and 10 MRC units per chick. In the absence of added calcium, responses occur to similar threshold and maximal doses of hormone, but the slope of the log dose-response line is too low for a satisfactory assay. In view of evidence presented elsewhere that calcium enters bone cells following injection of PTH and may act together with cyclic AMP as a second messenger in the skeletal response, we believe that the transient hypercalcemia caused by calcium injection enhances the response to iv PTH by increasing the quantity of calcium ions entering cells during a limited period of increased membrane permeability. (Endocrinology92: 454, 1973)