IL-11, initially described in 1990 as a bone marrow siroma-derived hematopoietic cytokine, is a highly conserved. 178 amino acid cationic protein thought to exist as a four-helix bundle. The human and the non-human primate genes for IL-11 have been localized to the long arm of chromosome 19 at band 19ql3.3-q 13.4. IL-11 is a member of the IL-6 ligand family since it utilizes gpl30 as the signaling component of its receptor, and amino acid residues 59 (methionine), 41 (lysine), and 98 (lysine) are critical for the function of the IL-11 protein, being completely conserved in the mouse, nonhuman primate, and human.IL-11 has multiple activities that continue to be characterized. Initially, in vitro and in vivo studies demonstrated a hematopoietic activity, largely manifest as thrombopoiesis in humans. rhIL-11 has been developed and recently approved by the US Food and Drug Administration for use in the prevention of severe thrombocytopenia occurring after cancer chemotherapy.Studies of cells and tissues from other organ systems indicate that IL-11 is produced by a variety of other cell types and has activity in protection and restoration of the gastrointestinal mucosa, major effects as an immunomodulaling agent, and activity in bone metabolism. Developmental investigations in mice indicate a widespread distribution of IL-11 expression in the embryo. Of great interest is the finding, from IL-11 receptor a chain knockout mice, that IL-11 signaling is an absolute requirement for normal development of placenlalion and survival to birth.