The deletion of MCD4 leads to an increase in beta-1,6-glucan level and a decrease in glycosylphosphatidylinositol-anchored protein and mannan levels in the cell wall of Saccharomyces cerevisiae, suggesting that mcd4 deletion mutant (mcd4 Delta) displays beta-glucans on the cell surface without a mannan cover. An observation of the cell surface of mcd4A cells and an examination of the effect of contact between mcd4 Delta cells and mouse macrophages indicated that macrophages were activated by contact with mcd4 Delta cells displaying beta-glucans on the cell surface. We further examined the effect of intraperitoneal ethanol-fixed mcd4 Delta cells on the survival period of mice infected with Candida albicans. mcd4A cells prolonged the survival period, implying that mcd4 Delta cells may enhance the immune function of mice via macrophage activation. Moreover, we examined the structures of beta-glucans (i.e., alkali- and acetic acid-insoluble beta-glucans) extracted from mcd4 Delta with C-13-NMR and the effect of extracted beta-glucans on TNF-alpha secretion from macrophages. The structures of the beta-glucans from mcd4 Delta differed from those of wild type (WT); however, there was no difference in tumor necrosis factor-alpha (TNF-alpha) secretion level between beta-glucans from mcd4 Delta and those from WT. The yield of purified beta-glucans obtained from dry cells of mcd4 Delta was higher than that obtained from dry cells of WT. mcd4 Delta may be a superior strain for the preparation of beta-glucans.