${eta}-Glucan$, one of the major cell wall components of Saccharomyces cerevisiae, is known to enhance the immune function, especially by activating macrophages. Accordingly, in an effort to develop a safe and efficient immune stimulatory agent, we prepared crude ${eta}-glucan$ (glucan-p1) and partially purified ${eta}-glucan$ that was free of mannoproteins (glucan-p2), and evaluated their effect on both the macrophage function and resistance to E. coli-induced peritonitis. To investigate the function of the macrophages, phagocytosis, $TNF-{alpha}$ secretion, oxygen burst, and the expression of cytokine genes such as $IFN-{gamma}$ and IL-12 were analyzed. Glucan-p2 markedly stimulated the macrophages with all these parameters. Glucan-p1, however, did not stimulate phagocytosis, yet it induced $TNF-{alpha}$ secretion, oxygen burst, and the expression of $IFN-{gamma}$ and IL-12, although less efficiently than glucan-p2. Finally, to test the in vivo protective effect of {eta}-glucan against infection, the survival of mice from E. coli-induced peritonitis was investigated. After 24 h of the peritoneal challenge of E. coli, all of the mice treated with glucan-p2 survived whereas none survived in the control group. Glucan-p1 showed only a marginal effect in protecting the mice. These results suggest that mannoprotein-free gluean-p2, but not gluean-p1, can serve as an effective immune-stimulating agent.