S ecretion by tumor cells of circulating bone-resorbing factors may frequently underlie the hypercalcemia that occurs in patients with malignancy. Efforts to identify the responsible mediators have been hampered by a lack of available human tumor cell systems suitable for study of the pathogenesis of the humoral hypercalcemia syndrome. We have established a transitional-cell carcinoma (TCC) line in vitro from a patient with humoral hypercalcemia. These cells are tumorigenic and cause hypercalcemia in athymic nude mice. Culture medium conditioned by TCC cells contains potent bone-resorbing activity in vitro, the physical and biological properties of which are similar to those of bone-resorbing activity present in the original patient's urine. The bone-resorbing activity of the TCC factor is accompanied by increased prostaglandin release from bone and is blocked by indomethacin and calcitonin. The TCC-derived bone-resorbing activity coelutes with prostaglandin-stimulating activity during gel filtration with an approximate molecular weight of 15,000. This activity is nondialyzable, stable to concentrated urea and reducing agents, and destroyed by boiling. The TCC factor does not increase cyclic AMP production in bone or kidney bioassays and does not exhibit transforming growth factor activity. We conclude that a unique macromolecular factor released by TCC cells causes bone resorption by a mechanism dependent upon stimulation of bone cell cyclooxygenase, and that this factor is the probable cause of the hypercalcemia in vivo. The TCC cell line provides a new model for study of the human humoral hypercalcemia syndrome.