We have demonstrated interleukin-6 (IL-6) production by human renal carcinoma cells. The IL-6 gene expression was detected by Northern blot analysis in 22 of 43 primary renal cell carcinoma tissues and in five of seven renal cell carcinoma cell lines. Immunohistochemical analysis confirmed the expression of IL-6 by the tumor cells. Patients with a high-level expression of IL-6 had significantly greater incidences of lymph node metastasis and a larger increase in serum C-reactive protein than those without it. We have also probed for the presence of IL-6 receptor by Northern blot analysis; we detected this receptor in 11 of the 43 primary renal cell carcinoma tissues but in none of the seven renal cell carcinoma cell lines. However, by use of the complementary DNA-polymerase chain reaction, the IL-6 receptor transcript was detected in all specimens, including the seven cell lines. No expression of the interleukin-3 (IL-3) gene was identified in any of the 43 primary renal cell tumors. These data provide evidence that IL-6 and its receptor may play a role in promoting the transformation and/or proliferation of renal cell carcinomas as well as in teh development of symptoms.