Interleukin (IL)-11 is a bone marrow fibroblast derived cytokine with a wide spectrum of activities in different biological systems. It has been shown that IL-11 supports the growth of certain types of plasmacytoma and hybridoma cells, enhances antigen-specific antibody responses, synergizes with IL-3 in supporting megakaryocyte colony formation, acts synergistically with IL-3 in shortening the G0 period of early progenitors, induces the synthesis of acute phase proteins, and inhibits lipoprotein lipase activity and adipocyte differentiation. The human IL-11 gene, which is localized at 19q13.3-13.4, consists of five exons and four introns. Initial biochemical characterization has identified a 151 kDa protein as the potential IL-11 binding subunit of the receptor complex. Because of the overlapping biological activities between IL-6 and IL-11, we compared the signal transduction pathways mediated by IL-6 or IL-11 in cell lines responsive to both cytokines. Results from protein tyrosine phosphorylation and immediate response gene expression suggest that there are convergent and divergent points along the signal transduction pathways utilized by IL-6 or IL-11. The IL-6 signal transducer, gp130, appears to be involved in the IL-11 mediated signaling. Other cytokines such as leukemia inhibitory factor, oncostatin M and ciliary neurotrophic factor have also been shown to utilize gp130 as a signal transducer. The significance of growth factor sharing common biological activities and signaling pathways will be discussed.