Toll-like receptor-4 activation by lipopolysaccharide (LPS) induces the expression of interferon-β (IFN-β) in a MyD88-independent manner. Here we report that mice devoid of the JAK protein tyrosine kinase family member, Tyk2, were resistant to shock induced by high doses of LPS. Basal and LPS-induced expression of IFN-β and IFN-α4 mRNA in Tyk2-null macrophages were diminished. However, Tyk2-null mice showed normal systemic production of nitric oxide and proinflammatory cytokines and the in vivo response to tumor necrosis factor (TNF) was unperturbed. IFN-β–null but not STAT1-null mice were also resistant to high dose LPS treatment. Together, these data suggest that Tyk2 and IFN-β are essential effectors in LPS induced lethality.