Interleukin-6 (IL-6) was found to be a pleiotropic cytokine which plays important roles not only in immune response but also in inflammation and hematopoiesis since the cDNA for IL-6 was originally cloned as a B cell differentiation factor (BCDF). IL-11, leukemia inhibitory factor, oncostatin M, and ciliary neurotrophic factor have similar effects with IL-6. This functional redundancy was well explained by the common signal transducer gp 130. Furthermore, recent studies have revealed the mechanisms to regulate the gene expression of IL-6 and to transduce the signal through IL-6 receptor system. On the basis of these results, elucidation of pathogenic significances of IL-6 and new therapeutic approaches in various diseases are now expected.