IL-11 is a unique growth factor derived from cells making up the HM. Although cloned based on IL-6-like bioactivity, IL-11 and IL-6 have distinct biologic profiles (Table 1). IL-11, like many recently cloned growth factors, has pleiotropic effects on hematopoietic cells presumably depending on the cytokine and cellular environment into which it is introduced. However, some general findings are consistent (Table 2). In addition, IL-11 has significant effects, either primary or secondary, on nonhematopoietic cells, including neurons, small intestine crypt progenitor/stem cells, and preadipocytes. The institution of human trials with IL-11 will provide important information on the pharmacologic effects of IL-11 on human hematopoietic cells in the context of frequently used chemotherapy protocols. The physiologic role(s) of IL-11 are unknown but will become clear (at least in the mouse) with gene targeting experiments underway in several laboratories.