Hypocalcin, the major hormone with hypocalcaemic action in fish, was isolated from trout corpuscles of Stannius (SCs). The bioactivity of hypocalcin was assessed in a parathyroid hormone (PTH) bioassay involving bone resorption in embryonic mouse calvaria. Calcium and phosphate release and lactate production were stimulated in a dose-dependent manner by hypocalcin. On a molar basis about equal amounts of hypocalcin and PTH were required to obtain similar effects in this assay. Hypocalcin did not stimulate cyclic AMP production either in mouse calvaria or in cultured osteoblasts. In this respect hypocalcin resembles shortened or N-terminus-modified PTH molecules that induce bone resorption without increasing cyclic AMP levels. Since hypocalcin and PTH have comparable bioactivity in this mammalian bioassay (as well as in fish bioassays), we tentatively suggest that both hormones are structurally similar and that both hormones may act via the same receptors. The two hormones show no resemblance to one another in primary structure, so we suggest that they have similarities in tertiary structure.