The leukemia inhibitory factor (LIF) belongs to the interleukin-6 (IL-6) cytokine family, which is involved in various biological processes including inflammation, immune responses, and embryonic development. Leukemia inhibitory factor is a glycoprotein growth and differentiation factor with pleiotropic activity. LIF has potent effects on the hematopoietic system, including megakaryocyte progenitor cells. In addition, LIF has bone regeneration activity, induces cachexia and acute-phase response in hepatocytes, and inhibits adipogenesis, to mention the more important activities. In vivo LIF treatment in monkeys and rodents was followed by signs of general toxicity, cachexia, acute-phase reaction, and stimulation of hematopoiesis. The safety margin for possible therapeutic effects on hematopoiesis seems to be very narrow.