β-Glucans stimulate leukocyte anti-infective activity, enhance murine hematopoietic recovery following bone marrow injury and mobilize murine progenitor cells from bone marrow. This study evaluated the in vitro hematopoietic potential of the β-glucan, PGG-glucan, on human bone marrow mononuclear cells (BMMC) and CD34+ BMMC compared with protein cytokines. In the presence of submaximal concentrations of recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF; 0.5 ng/ml), PGG-glucan significantly increased BMMC myeloid colony formation comparable to the increase observed with either interleukin-3 (rhIL-3) or stem cell factor (rhSCF). Moreover, the addition of PGG-glucan to cultures containing GM-CSF + IL-3 or GM-CSF + SCF significantly augmented granulocyte-macrophage colony production above baseline, demonstrating that PGG-glucan acts independently of those early-acting cytokines and can enhance their activity in an additive manner. Anti-PGG-glucan monoclonal antibody specifically abrogated the growth-enhancing effect of added PGG-glucan in a saturable manner and other control carbohydrate polymers failed to affect colony formation. Further, PGG-glucan was not associated with induction of IL-6, GM-CSF production and removal of accessory cells by CD34+ cell isolation did not alter the PGG-glucan effect. These data demonstrate that PGG-glucan acts on committed myeloid progenitors to enhance human hematopoietic activity by a mechanism of direct action independent of IL-3 or SCF and independent of secondary cytokine stimulation.