BACKGROUND: Activation of neutrophil is critical for the clearance of microorganisms and toxic host mediators during sepsis. Unfortunately the activated neutrophil and its toxic byproducts can produce tissue injury and organ dysfunction. The leucocyte CD11/18 adhesion complex regulates neutrophil-endothelial cell adhesion, the first step in neutrophil migration to sites of infection and inflammation. To investigate the potential of neutrophil inhibition as a treatment strategy for sepsis, we evaluated the effects of monoclonal antibody against CD11b (MAb 1B6) in rats intrabronchial challenged with Escherichia coli. METHODS: Animals were randomly assigned to receive monoclonal antibody against CD11b (1 mg/kg, sc) and bovine serum albumin(BSA, 1 mg/kg, sc) 6 hr before, at 0 and 6 hr after intrabronchial challenge of 20xl0(9) CFU/kg E. coli 0111. Animals were randomized to treat either 24, 60 or 90% oxygen after bacterial challenge and begining 4 hr after inoculation, all animals were received 100 mg/kg ceftriaxone qd for 3 days. Peripheral and alveolar neutrophil(by bronchoalveolar lavage) counts and lung injury parameters such as alveolar-arte rial PC2 difference, wet to dry lung weight ratio and protein concentration of alveolar fluid were measured in survived rats at 12 hr and 96 hr. RESULTS: Monoclonal antibody against CD11b decreased circulating and alveolar neutrophil especially more in 12 hr than in 96 hr. The lung injury parameters of antibody-treated animals were not different from those of BSA-treated animals, but It was meaningless due to small number of survived animals. The early(6 hr) mortality rate was significantly increased in antibody-treated group(51%) compared to BSA-treated group(31%)(P = 0.02) but late(from 12 hr to 72 hr) mortality rate was not different in antibody-treated group(44%) from BSA-treated group(36%)(P = 0.089). CONCLUSION: Leucocyte CD11b/18 adhesion molecule is known to regulate neutrophil migration to the site of infection and inflammation. The monoclonal antibody against CD11b decreased alveolar neutrophil in rats with pulmonary sepsis and increased early mortality rate. Therefore, we can speculate that monoclonal antibody against CD11b blocks of alveolar recruitment of neutrophils, impairs host defense mechanism and increases early mortality rate of pulmonary sepsis in rat.