IL-10 is an important anti-inflammatory cytokine that can inhibit the production of many pro-inflammatory cytokines. Both human and animal studies have shown that pro-inflammatory cytokines play an important role in pneumonia and other inflammatory lung diseases. In the present study, IL-10 knockout (KO) and wild-type mice were infected with Corynebacterium kutscheri to determine whether the severity of pathogenesis and whether protective immunity could be altered in the absence of IL- 10. The survival rate was significantly lower in IL-10 KO mice than wild-type mice. The number of neutrophils in bronchoalveolar lavage fluid and blood were found to be higher in IL-10 KO mice than wild-type mice. IL-10 KO mice showed greater neutrophil infiltration, excessive inflammation, and weight-loss compared with wild-type mice. Furthermore, upregulation of IFN-gamma in bronchoalveolar lavage fluid, and upregulation of MIP-1alpha and IP-10 mRNA in the lungs of IL-10 KO mice compared with wild-type mice after C. kutscheri infection were observed. These results suggest that IL-10 plays an important role in the anti-inflammatory properties against C. kutscheri infection, and that lack of IL-10 leads to a more severe pulmonary inflammatory response. This increased susceptibility to C. kutscheri pneumonia is at least in part caused by IL-10 deficiency and severe recruitment of neutrophils.