The effects of interleukin-6 (IL-6) in vivo were assessed by inoculating Chinese hamster ovarian (CHO) cells which were transfected with the murine IL-6 gene in nude mice. Nude mice bearing CHO cells expressing IL-6 developed hypercalcemia. Tumor-bearing mice also showed increases in white cell count, platelet count, and decreases in body weight. In nude mice carrying CHO tumors which had not been transfected with the IL-6 gene, there were no changes in these parameters. These results suggest that increased circulating concentrations of IL-6 in patients with malignant disease may contribute to a number of paraneoplastic syndromes including hypercalcemia, cachexia, leukocytosis and thrombocytosis.