We used a Chinese hamster ovary cell line that had been transfected with the human tumor necrosis factor (TNF) gene and constitutively produced TNF when transplanted in nude mice to study the effects of continuous production of TNF on calcium homeostasis. Continuous exposure to TNF caused increased osteoclastic bone resorption and humoral hypercalcemia in these animals. The mice bearing TNF-producing tumors were significantly hypercalcuric compared to mice bearing control tumors, but urinary cAMP excretion was unchanged. Mice bearing Chinese hamster ovary cell tumors containing the empty vector did not demonstrate hypercalcemia or increased bone resorption. This model system using transfected cells to continuously produce cytokines in vivo is more analogous to the pathophysiological conditions present in patients than intermittent injections and can produce much longer exposures than infusion pumps. Such model systems should allow a better understanding of the role of factors involved in humoral hypercalcemia.