SUMMARY A parallel-line bioassay for parathyroid hormone based on the release of stable calcium from whole mouse calvaria in tissue culture has been developed from the method of Zanelli, Lea & Nisbet (1969). There has been no change in either the dose range (0·06–1·2 units/2 ml culture) or the bioassay statistics over a prolonged working period but the total assay time has been shortened and the preparative stages made easier.