Interleukin 6, an essential factor for B cells to differentiate to immunoglobulin-producing cells has been demonstrated to be involved in abnormal polyclonal plasmacytosis in cardiac myxoma. Castleman's disease, rheumatoid arthritis and AIDS. Furthermore, IL-6 was found to be a potent growth factor for both plasmacytoma and myeloma. The involvement of IL-6 in plasma cell abnormalities was further demonstrated by the generation of a lethal plasma cell growth in IL-6 transgenic mice. Continuous IL-6 gene expression could be considered to play an essential role in both polyclonal plasmacytosis and the generation of plasma cell neoplasias.