Potent Toll-like receptor 4 (TLR4) activation by endotoxin has been intensely studied, but the molecular requirements for endotoxin interaction with TLR4 are still incompletely defined. Ligand-receptor interactions involving endotoxin and TLR4 were characterized using monomeric endotoxin.protein complexes of high specific radioactivity. The binding of endotoxin.MD-2 to the TLR4 ectodomain (TLR4ECD) and transfer of endotoxin from CD14 to MD-2/TLR4ECD were demonstrated using HEK293T-conditioned medium containing TLR4ECD+/-MD-2. These interactions are specific, of high affinity (KD<300 pm), and consistent with the molecular requirements for potent cell activation by endotoxin. Both reactions result in the formation of a Mr approximately 190,000 complex composed of endotoxin, MD-2, and TLR4ECD. CD14 facilitates transfer of endotoxin to MD-2 (TLR4) but is not a stable component of the endotoxin.MD-2/TLR4 complex. The ability to assay specific high affinity interactions of monomeric endotoxin.protein complexes with TLR4ECD should allow better definition of the structural requirements for endotoxin-induced TLR4 activation.