Interleukin (IL)-11 is a bone marrow fibroblast derived cytokine with a wide spectrum of in vitro biological activities in the hematopoietic, lymphopoietic, hepatic, adipose, neuronal and osteoclast systems, either alone or in synergy with other hematopoietic growth factors. In vivo administration of IL-11 in mice, rats and nonhuman primates has demonstrated the thrombopoietic effects of this cytokine. The expression of the human IL-11 gene, which is localized at 19q13.3-13.4, can be controlled at both the transcriptional and post-transcriptional levels. Initial biochemical characterization has identified a 151 kD protein as the potential IL-11 binding subunit of the receptor complex. Like other cytokines such as IL-6, leukemia inhibitory factor (LIF), oncostatin M (ONC) and ciliary neurotrophic factor (CNTF), IL-11 has been shown to utilize IL-6 signal transducer, gp130. Because of the overlapping biological activities, the similarities in the predicted tertiary structures, and the sharing of common signal transducer protein, we have compared the signal transduction pathways mediated by these cytokines in various cell types. Studies of protein tyrosine phosphorylation, primary response gene expression and signaling molecules known to be important in transducing mitogenic signals have suggested that there are convergent and divergent points along the signal transduction pathways utilized by IL-11, IL-6, LIF and ONC. These observations may explain the biological pleiotropy and redundancy of this group of cytokines.