The use of calcitonin (CT) is established as a treatment of Paget's disease of bone and postmenopausal osteoporosis (PMO). Salmon calcitonin (sCT), which differs in 14 of the 32 amino acids from human calcitonin, has found a wider distribution world wide, although antibody formation against sCT has been reported in more than 70% of the patients on continuous sCT treatment. The clinical significance of these antibodies has been discussed controversially, because the occurrence of antibodies is not always associated with the development of secondary resistance. Using an in vitro bioassay, based on the CT-mediated increase of the cyclic AMP (cAMP) production of the human breast cancer cell line T 47 D we could identify a neutralizing activity against sCT in the serum of a subset of patients with formation of antibodies against sCT which was related to the development of secondary resistance. Antibody formation against human calcitonin (hCT) has been reported only once before. Binding and neutralizing antibodies were now observed in 1 of 33 patients with PMO treated with hCT. Due to a low neutralizing activity, clinical sequelae were not to be expected in this patient. The formation of neutralizing antibodies against calcitonin is common after treatment with salmon but a rare phenomenon after treatment with human calcitonin. We recommend monitoring of patients with postmenopausal osteoporosis and Paget's disease of bone on long term treatment with sCT or hCT for neutralizing antibody formation in order to evaluate the therapeutic effect of treatment.