β-Glucan is a polysaccharide expressed on the cell walls of fungi. It is known that β-glucan is recognized by a family of C-type lectin receptors, dectin-1, which is expressed mainly on myeloid immune cells, including macrophages, neutrophils and dendritic cells. Raw 264.7 cells were treated with β-glucan from Schizophyllum commune. β-Glucan was not cytotoxic up to 400 μg/mL as measured by MTT assay. To measure the activity of macrophages, NO and TNF-α assays were performed in Raw 264.7 cells. Treatment with β-glucan for 24 hr significantly increased production of NO and TNF-α compared with control groups (p<0.05), indicating activation of macrophages. To measure inhibition of breast cancer cell proliferation, MTT assay was performed in MDA-MB-231 cells. Cell viability was significantly decreased in the group treated with 400 μg/mL of βglucan for 48 hr (p<0.05) compared to the control group. However, tumor volume was decreased in the groups administered 200 μg of β-glucan/mouse compared to the control group. These results indicate that β-glucan inhibits breast cancer cell growth through the induction of apoptosis.