To investigate the correlation between interleukin-6 and urothelial neoplasms, interleukin-6 activities in blood and urine samples of patients with bladder carcinoma were measured with a proliferation assay using an interleukin-6 dependent murine hybridoma clone, MH60.BSF2. A total of 43 patients and 15 normal volunteers were entered into this study. All of the patients were examined preoperatively and 26 were reexamined more than 6 days postoperatively to eliminate the effect of surgical injury on interleukin-6 secretion. The interleukin-6 titers in urine and serum increased in accordance with the progression of the tumor stage, and tumor removal induced a remarkable decrease in the titer of urinary interleukin-6. Although the interleukin-6-producing site has not been elucidated yet, our study suggests that interleukin-6 activity in bladder carcinoma patients may reflect the immunoreaction against the tumor in local urothelium.