Leukemia inhibitory factor (LIF) is a cytokine active in vitro on different target cells. It is detected in vivo during mouse gestation in both extraembryonic membranes and maternal tissues. Two isoforms have been described maintaining embryonic stem cells in culture in a pluripotent state. However, overexpression of their cDNAs in chimeric mouse embryos observed between 6.5 and 9.5 days postcoitus gave strikingly different phenotypes. Embryos overexpressing the diffusible form of LIF cDNA looked essentially normal. Chimerae expressing LIF associated with the extracellular matrix cDNA showed an abnormal proliferation of tissues and the absence of differentiated mesoderm. They have not undertaken the normal pathway of gastrulation.