LEUKAEMIA inhibitory factor (LIF) has a variety of effects on different cell types in vitro¹, inhibiting the differentiation of embryonic stem cells^2,3 and promoting the survival and/or proliferation of primitive haematopoietic precursors^4,5 and primordial germ cells^6,7. Here we show that LIF-deficient mice derived by gene targeting techniques have dramatically decreased numbers of stem cells in spleen and bone marrow. Injection of spleen and marrow cells from these mice promotes long-term survival of lethally irradiated wild-type animals, however, showing that the LIF⁻ stem cells remain pluripotent. The numbers of committed progenitors are also reduced in the spleen but not the bone marrow, suggesting that stem cells interact differently with the splenic and medullary microenvironment. Heterozygous animals are intermediate in phenotype, implying that LIF has a dosage effect, and defects in stem cell number can be compensated by exogenous LIF. LIF thus appears to be required for the survival of the normal pool of stem cells, but not their terminal differentiation.