This study was designed to determine the role of interleukin (IL)-1 in the inflammatory response against experimentally induced pneumonia caused by Klebsiella pneumoniae. The host immune responses of IL-1 gene knockout (IL-1 KO) mice and immunocompetent wild-type (WT) mice were compared after pulmonary infection with K. pneumoniae. There were no significant differences between the survival rates and viable bacterial counts in lungs and blood of IL-1 KO and WT mice after pulmonary infections under different conditions. Histopathological analysis showed a similar inflammatory response in both groups of mice. However, in the early stage of infection, the level of tumour necrosis factor alpha (TNF-alpha) in homogenized lungs of IL-1 KO mice was significantly higher than in WT mice. To determine the role of endogenous TNF-alpha in the recovery of the defence mechanism in IL-1 KO mice, mice were treated with an anti-TNF-alpha mAb before infection with K. pneumoniae. The results revealed a significantly lower survival rate of anti-TNF-alpha mAb-treated IL-1 KO mice than BSA-treated IL-1 KO mice. The data suggest that compensatory production of TNF-alpha in IL-1 KO mice contributes to the host defence against K. pneumoniae infection.