An invading pathogen must be held in check by the innate immune system until a specific immune response can be mounted. In the case of Gram-negative bacteria, the principal stimulator of the innate immune system is Lipopolysaccharide (LPS), a component of the bacterial outer membrane(1). In vitro, LPS is bound by lipopolysaccharide-binding protein (LBP)(2) and transferred to CD14-the LPS receptor on the macrophage surface(3,4)-or to high-density lipoprotein (HDL) particles(5,6). Transfer to CD14 triggers an inflammatory response which is crucial for keeping an infection under control. Here we investigate how LBP functions in vivo by using LBP-deficient mice. Surprisingly, we find that LBP is not required in vivo for the clearance of LPS from the circulation, but is essential for the rapid induction of an inflammatory response by small amounts of LPS or Gram-negative bacteria and for survival of an intraperitoneal Salmonella infection.