Objective:To investigate the mechanism of polymyxin B(PMB)antagonizing the biological activity of lipopolysaccharide(LPS). Methods:The affinity of PMB for LPS and lipid A was assayed by biosensor,and the neutralization of PMB for LPS(2 ng/ml)was detected by kinetic turbidimetric limulus test.The releases of TNF-αand IL-6 in murine peritoneal macrophages(PM(?)) after exposure to LPS(100 ng/ml)were detected,and the expression levels of TLR4, TNF-αand IL-6 mRNA in PM(?) induced by LPS(100 ng/ml)were measured by RT-PCR. Results:PMB had high-affinity to LPS and lipid A with dissociation equilibrium constants of 18.9 nmol/L and 11.1 nmol/L,respectively,and neutralized LPS in a dose- dependent manner.Furthermore,PMB could markedly inhibit the expressions of TLR4,TNF-αand IL-6 mRNA and the release of cycokines in LPS-stimulated murine PM(?). Conclusions:PMB neutralizes LPS and inhibites the expression and release of cycokines in macrophages,in which the affinity of PMB for lipid A plays an important role. Objective: To investigate the mechanism of polymyxin B (PMB) antagonizing the biological activity of lipopolysaccharide (LPS). Methods: The affinity of PMB for LPS and lipid A was assayed by biosensor, and the neutralization of PMB for LPS (2 ng / ml ) was detected by kinetic turbidimetric limulus test. These releases of TNF-α and IL-6 in murine peritoneal macrophages (PM (?)) after exposure to LPS (100 ng / ml) were detected, and the expression levels of TLR4, Results: PMB had high-affinity to LPS and lipid A with dissociation equilibrium constants of 18.9 nmol / L and 11.1 nmol / L, respectively, and neutralized LPS in a dose-dependent manner. Stillmore, PMB could markedly inhibit the expressions of TLR4, TNF-αand IL-6 mRNA and the release of cycokines in LPS-stimulated murine PM PMB neutralizes LPS and inhibites the expression and release of cycokines in macrophages, in which the affinity of PMB for lipid A plays an important role.