IL-11 is a pleiotropic cytokine originally isolated from a bone marrow stromal cell line. It has been shown to share many activities with IL-6, namely to stimulate T cell-dependent B cell maturation, megakaryopoiesis and various stages of myeloid differentiation, but to inhibit adipogenesis. However, the activity of IL-11 on different stages of erythropoiesis in vitro clearly sets it apart form IL-6. IL-11 has little hematopoietic colony stimulatory activity of its own although it sustains terminal differentiation of the late erythroid progenitors CFU-E. In combination with IL-3, IL-11 has profound stimulatory effects on early multipotent hematopoietic progenitors (pre-CFCmulti), on multilineage colony-forming cells (CFCmulti), as well as on erythroid progenitors. The combination of IL-11 with the ligand for c-kit (KL) preferentially acts on early cells since it promotes the multiplication of pre-CFCmulti and stimulates highly proliferative erythroid progenitors that yields remarkable macroscopic erythroblast colonies in culture. The synergistic activity of IL-11 and KL, two stromal factors present in the bone marrow microenvironment, points to a pivotal role of IL-11 in early hematopoiesis. In vivo administration of recombinant human IL-11 elevates the number of circulating neutrophils and platelets and increased megakaryopoiesis in normal mice and primates.