The human epidermoid bronchial carcinoma (BEN) cell line has been shown to have specific membrane binding sites for calcitonin and to secrete high-molecular-weight forms (ranging from 40000 to 10000) of immunoreactive calcitonin. Synthetic salmon and human calcitonins and a thyroid extract of porcine calcitonin have been shown to displace 125I-labelled salmon calcitonin from the receptors in a dose-related fashion. The binding to these receptors of calcitonins derived from the BEN cell line and a medullary thyroid carcinoma with molecular weights ranging from 28000 to 3500 (both separated by gel-filtration chromatography) has been investigated. Neither major peaks of BEN-cell-line calcitonin showed receptor binding activity. Only one form of medullary thyroid carcinoma calcitonin, that which co-eluted with synthetic calcitonin monomer on gel-filtration chromatography, caused any significant displacement of labelled hormone from the receptors.