Biologically active PTH (bio-PTH) has been assayed by the cytochemical bioassay in five members (three affected) of a family with typical features of familial hypocalciuric hypercalcaemia (FHH). Carboxy-terminal immunoreactive PTH was undetectable and bio-PTH was within the normal range in all the subjects regardless of whether or not they were hypercalcaemic. These results suggest that increased biological activity of circulating PTH cannot account for the hypercalcaemia of FHH.