Interleukin 6 (IL-6) is a pleiotropic cytokine, with a wide range of biological effects. The diverse biological actions of IL-6 could play important roles in the enhancement or suppression of tumor growth and development. IL-6 has been seen to act as an autocrine and/or paracrine growth factor for various human tumors, including multiple myeloma, renal cancer, and AIDS-associated Kaposi's sarcoma. However, IL-6 also can exert potent anti-tumor effects: administration of IL-6 has been seen to result in decreased tumor appearance in experimental animal systems. Therefore, potentially useful anti-tumor therapeutic strategies could include the inhibition of the activity of IL-6, or alternatively, the enhancement of anti-tumor responses by the administration of exogenous IL-6.