It has been demonstrated that interleukin-6 (IL-6) is a cytokine regulating immune response, acute-phase reaction and hematopoiesis. The deregulated expression of IL-6 was suggested to be actually involved in the pathogenesis of polyclonal B cell activation and autoimmune diseases such as rheumatoid arthritis. It could be hypothesized that continuous polyclonal B cell activation may be eventually leading to the generation of plasmacytoma/myeloma, possibly with additional expression of oncogene(s) such as c-myc gene. Therefore, future studies on the gene regulation of IL-6 would provide critical informations on the molecular pathogenesis of these diseases. Furthermore, IL-6 could be used as anti-cancer drug in certain tumors. Moreover, inhibitors of IL-6 such as anti-IL-6 monoclonal antibodies or soluble forms of IL-6 receptors could be useful in the treatment of such polyclonal/monoclonal B cell abnormalities.