Evidence is presented that a tumor-derived transforming growth factor is responsible for stimulating bone resorption and causing hypercalcemia in an animal tumor model of the hypercalcemia of malignancy. Both conditioned medium harvested from cultured tumor cells and tumor extracts of the transplantable rat Leydig cell tumor associated with hypercalcemia contained a macromolecular bone resorbing factor with the chemical characteristics of a tumor-derived transforming growth factor.