Klebsiella pneumoniae is a common virulent causative agent for pneumonia. Lysozyme has previously been shown to play an important role in nonimmune host defense of the airways. This study was undertaken to assess the role of lysozyme M, the major isoform of lysozyme in mouse lung, in the killing of K. pneumoniae in lysozyme M^−/− mice and transgenic mice with increased expression of lysozyme (lysozyme^tg mice). The airways of lysozyme M^−/− mice maintained in a pathogen-free facility were colonized by Lactobacilli, a component of the oropharyngeal flora. No lactobacilli were detected in the lungs of wild-type (WT) or lysozyme^tg mice. Twenty-four hours after intratracheal infection with K. pneumoniae, bacterial killing was enhanced 9-fold in lysozyme^tg mice compared with WT mice and 43-fold compared with lysozyme M^−/− mice. In survival studies, no lysozyme M^−/− mice survived beyond 72 hours after infection, whereas 75% of lysozyme^tg (p < 0.01) and 25% of WT mice survived to 120 hours (p < 0.01). Deficiency of lysozyme M in the lungs increased susceptibility to K. pneumoniae infection, whereas increased expression of lysozyme conferred resistance to infection and enhanced survival.