Leukaemia inhibitory factor （LIF） is produced by many different cell types and has pleiotropic actions. LIF stimulates the differentiation of the macrophage cell line M1, and the proliferation of haematopoietic stem cells. In vivo, it has profound effects on haematopoiesis, particularly in combination with other cytokines such as IL-3, causing increased platelet formation. LIF allows embryonic stem cells to remain in an undifferentiated state and can maintain their proliferation in culture. LIF stimulates synthesis of acute phase proteins by liver cells, increases bone resorption, stimulates differentiation of cholinergic nerves and loss of body fat.