Interleukin 6 (IL-6)/B cell stimulatory factor 2 is a multifunctional cytokine produced by both lymphoid and nonlymphoid cells. IL-6 regulates immune response, acute phase reaction, and hematopoiesis. It was found that IL-6 production by T cells is dependent on macrophages, and IL-6 is one of essential factors for pokeweed mitogen induced immunoglobulin production. Both high- and low-affinity IL-6 receptors were identified. The molecular cloning of IL-6 receptors demonstrated that this receptor is a member of the immunoglobulin superfamily. The deregulated production of IL-6 is suggested to be involved in the pathogenesis of autoimmune diseases and in the development of multiple myeloma.