Lipopolysaccharide (LPS)-binding protein (LBP) and CD14 represent key elements in monocyte activation by LPS. The mean concentration of LBP was 18.1 microgram/mL in normal serum and 40-60 micrograms/mL in serum of patients with septic shock, independent of the fact that patients had gram-negative or other infections. Ten percent normal serum presented large concentrations of LPS (in the microgram range) to monocytes. Only when diluted 1:100 was LBP in plasma a limiting factor for monocyte activation, as measured by tumor necrosis factor (TNF) release. When LBP was depleted from serum with anti-LBP antibodies, the resulting serum did not support TNF release of monocytes upon LPS challenge. In conclusion, monocyte activation resulting in TNF secretion was related to LBP, which is abundantly present in normal serum, and elevated two to three times in patients with septic shock.