Leukemia inhibitory factor (LIF) is a glycoprotein able to enforce differentiation and/or suppress clonogenic self-renewal in a number of myeloid leukemic cell lines. When acting on normal embryonic stem cells, it has the opposite action of preventing differentiation commitment. LIF is not a proliferative factor when acting alone on normal hemopoietic cells, but can potentiate the action of interleukin 3 on blast cell and megakaryocyte precursors. When injected in vivo, LIF stimulates rises in megakaryocyte numbers and platelet levels. LIF also exhibits striking functional effects on a wide range of other cells including hepatic parenchymal cells, neurones, adipocytes, osteoblasts and gonadal cells. The polyfunctionality of LIF suggests strongly that it is normally intended to be produced locally and act as a local regulator. Despite its wide range of actions, LIF remains a promising candidate for clinical use in thrombocytopenia and myeloid leukemia.