Abstract: Objective To observe the effect of Klebsiella pneumoniae (Kle.p) on the expression of cytokines in mouse polymorphonuclear neutrophil granulocytes (PMNs). Methods Mouse models of acute pneumonia were induced by intranasal inoculation of 30 μl PBS containing 1×10⁷ Kle.p (heat-inactivated at 60 ℃ for 1.5 h).The mRNA and protein expressions of the cytokines in the isolated and purified PMNs were assayed by reverse transcriptional (RT)-PCR,Western blotting and enzyme- linked immunosorbent assay (ELISA),respectively. Results Kle.p infection induced the mRNA expressions of tumor necrosis factor (TNF)-α and interleukin (IL)-1β in the early stage,and TNF-α mRNA reached its peak at 6 h and IL-1β mRNA at 12 h after infection,both of which decreased 24 h after infection.Western blot analysis showed that the protein expressions of TNF-α and IL-1β were the highest at 6 h,and decreased obviously at 24 h after infection.ELISA showed that the secretion of TNF-α peaked at 6 h and decreased obviously at 24 h after infection,while the peak of IL-1β production occurred at 12 h and maintained the high level till 24 h after the infection. Conclusion TNF-α may participate in the immune defense of PMN against Kle.p infection mainly in the early stage of infection while IL-1β mainly in the later stage.