We previously showed that basic fibroblast growth factor (bFGF) is a potent mitogen for human bone marrow (BM) stromal cells and significantly delays their senescence. In the present study, we demonstrated that low concentrations of bFGF (0.2 to 2 ng/mL) enhance myelopoiesis in long-term human BM culture. Addition of bFGF to long-term BM cultures resulted in an increase in (a) the number of nonadherent cells (sixfold), particularly those of the neutrophil granulocyte series; (b) the number of nonadherent granulocyte colony-stimulating factor (G-CSF)- and granulocyte-macrophage colony-stimulating factor (GM-CSF)-responsive progenitor cells; (c) the number of adherent foci of hematopoietic cells (10-fold); and (d) the number of progenitor cells in the adherent stromal cell layer. These effects were not noted with higher concentrations of bFGF (20 ng/mL). Thus, low concentrations of bFGF effectively augment myelopoiesis in human long-term BM cultures, and bFGF may therefore be a regulator of the hematopoietic system in vitro and in vivo.