We studied the effect of the combination of rapid culture medium exchange with the addition of the human hematopoietic growth factors interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF), and erythropoietin (Epo) on the proliferation and differentiation of human long-term bone marrow cultures (LTBMCs). Individually and in combinations, IL-3, GM-CSF, and Epo were added to the culture medium of LTBMCs that were maintained with 50% medium volume exchange per day. The combination of IL-3 + GM-CSF + Epo generated the most prolific cultures with an order of magnitude increase in nonadherent cell production from weeks 2 through 8 in culture as compared with unsupplemented controls. Under these conditions, the cultures produced as many cells as were inoculated every 2 weeks and led to a greater than 2.5-fold expansion in terms of the number of nonadherent cells produced over a 6- to 8-week period. Furthermore, the LTBMCs produced nonadherent colony-forming unit-GM (CFU-GM) for more than 20 weeks. The rapid medium exchange combined with the addition of human hematopoietic CSFs significantly enhances the proliferation and differentiation of LTBMCs. These results indicate that addition of combinations of hematopoietic CSFs, together with a rapid medium exchange rate, can provide culture conditions that are suitable for the expansion of the progenitor cell pool and perhaps for the increased survival of hematopoietic stem cells in culture. Although these culture conditions still fall short of full reconstitution of functional human bone marrow, they provide an improved approach to hematopoietic cell culture that may permit the expansion and manipulation of progenitor cells in vitro.