Background: Cordyceps militaris have been reported to modify the immune and inflammatory responses both in vivo and in vitro. Macrophages play important roles in the innate immunity through the phagocytosis of antigens. This study examined the effects of Cordyceps militaris on the activation of murine macrophage RAW 264.7 cells and primary macrophages. Methods: The components contained in culture broth of Cordyceps militaris were purified by propyl alcohol extraction and HP 20 column chromatography to CMDB, CMDBW, CMDB5P, and CMDB25P. The amounts of nitric oxide (NO) were determined by using ELISA, Griess reagent respectively. The amounts of some cytokines were determined by using ELISA, western blot, and RT-PCR The expression levels of cell surface molecules (ICAM-1, B7-1 and B7-2) were measured by flow cytometric analysis. Results: All the components of Cordyceps militaris produced significant amounts of NO. In particular, CMDB produced much more NO in RAW 264.7 cells and primary macrophages than other fractions of Cordyceps militaris. CMDB increased significantly the production of tumor necrosis factor (TNF)-${alpha}$, interleukin (IL)-1${eta}$, and IL-6 dose-dependently in RAW 264.7 cells. Examination of the gene expression level also showed that the enhanced production of cytokines was correlated with the up-regulation of i-NOS expression, cycloxygenase (COX)-2 expression, IL-1${eta}$ and IL-6 expression, and TNF-${alpha}$ expression on the expression of mRNAs by semi-quantitative RT-PCR Western blot analysis also confirmed that CMDB enhances the expression level of these cytokines. Conclusion: These results show that CMDB stimulates the production of NO and pro-inflammatory cytokines and can also up-regulate the gene expression levels in macrophages.