The role of the capsule of Klebsiella pneumoniae in inducing cytokine production was investigated by comparing the responses of mice with experimentally induced pneumonia caused by capsulate (strain DT-S) or non-capsulate (mutant strain DT-X) K. pneumoniae. Anaesthetised ICR mice were inoculated intranasally. Whereas all DT-S-infected mice died within 3 days, no deaths were observed in DT-X-infected mice by 14 days after infection. During the early stage of infection, interferon-gamma (IFN-gamma) levels in bronchoalveolar lavage fluid (BALF) of DT-X-infected mice were significantly higher than those in DT-S-infected mice. In contrast, in the late stage of infection, serum levels of granulocyte macrophage-colony stimulating factor (GM-CSF) and IFN-gamma in DT-S-infected mice were significantly higher than those in DT-X-infected mice. Levels of interleukin10 (IL-10) in BALF and serum of DT-S-infected mice were significantly and persistently higher than those of DT-X-infected mice. The IL-10/TNF-alpha (tumour necrosis factor-a) ratios in BALF and serum indicated that higher levels of IL-10 production were induced in mice infected with strain DT-S than in those infected with strain DT-X. The results suggest that the capsule of K. pneumoniae may induce IL-10 production at the site of infection and, thereafter, these high IL-10 levels may serve to down-regulate the expression of pro-inflammatory cytokines.