TISSUE factor, a member of the cytokine-receptor superfamily and high-affinity receptor and cofactor for plasma factor VII/VIIa (ref, 1), is the primary cellular initiator of blood coagulation. It is involved in thrombosis and inflammation associated with sepsis, atherosclerosis and cancer(2), and can participate in other cellular processes including intracellular signalling(3), metastasis(4), tumour-associated angiogenesis(5), and embryogenesis(6). Here we report that inactivation of the tissue factor gene (TF) results in abnormal circulation from yolk sac to embryo beyond embryonic day 8.5, leading to embryo wasting and death, Vitelline vessels from null mice were deficient in smooth-muscle a-actin-expressing mesenchymal cells, which participate in organization of the vessel wall, This implies that tissue factor has a role in blood-vessel development.