To investigate the role of NF-IL6 in vivo, we have generated NF-IL6 (-/-) mice by gene targeting. NF-IL6 (-/-) mice were highly susceptible to infection by Listeria monocytogenes. Electron microscopic observation revealed the escape of a large number of pathogens from the phagosome to the cytoplasm in activated macrophages from NF-IL6 (-/-) mice. Furthermore, the tumor cytotoxicity of macrophages from NF-IL6 (-/-) mice was severely impaired. However, cytokines involved in macrophage activation, such as TNF and IFNγ, were induced normally in NF-IL6 (-/-) mice. Nitric oxide (NO) formation was induced to a similar extent in macrophages from both wild-type and NF-IL6 (-/-) mice. These results demonstrate the crucial role of NF-IL6 in macrophage bactericidal and tumoricidal activities as well as the existence of a NO-independent mechanism of these activities. We also demonstrate that NF-IL6 is essential for the induction of G-CSF in macrophages and fibroblasts.