Poly-(1,6)-beta-d-glucopyranosyl-(1,3)-beta-d-glucopyranose (PGG) beta-glucan is a soluble yeast-derived polysaccharide that has previously been shown to induce hematopoietic progenitor cell (HPC) mobilization. However, the mobilizing mechanism of action remains unknown. Here, we confirmed that PGG beta-glucan alone or in combination with granulocyte colony-stimulating factor (G-CSF) mobilizes HPC into the periphery. Optimal mobilizing effects were seen 24-48 hours after PGG beta-glucan doses of 4.8-9.6 mg/kg. Animals treated with G-CSF and PGG beta-glucan showed a collaborative effect in HPC mobilization compared with G-CSF treatment alone. Additional studies demonstrated that neither complement 3 nor complement receptor 3 played a role in this effect and that PGG beta-glucan treatment did not induce proinflammatory cytokine secretion. However, bone marrow cells from PGG beta-glucan-treated mice secreted abundant matrix metalloproteinase-9 (MMP-9), and PGG beta-glucan-induced HPC mobilization was abrogated in MMP-9 knockout mice. Moreover, we demonstrated that both hematopoietic and nonhematopoietic cells contributed to MMP-9 secretion upon PGG beta-glucan treatment. In addition, HPCs mobilized by PGG beta-glucan had similar levels of engraftment in host and lineage differentiation capability compared with those mobilized by G-CSF. Thus, PGG beta-glucan is an agent that enhances HPC mobilization and may improve the outcome of clinical stem cell transplantation.