Various growth factors are known to stimulate both early and late stages of human hematopoietic cell development in semisolid assay systems, but their role as microenvironmental regulators is poorly understood. To address this problem, we developed a novel coculture system in which highly purified primitive human hematopoietic cells were seeded onto an irradiated feeder layer of cells from a murine marrow-derived stromal cell line (M2–10B4) previously engineered by retroviral-mediated gene transfer to produce specific human factors. Effects on cells at very early, intermediate, and late stages of hematopoiesis were then evaluated by assessing the number of clonogenic cell precursors (long-term culture initiating cells [LTC-IC]), clonogenic cells, and mature granulocyte and macrophage progeny present in the cultures after 5 weeks. In the absence of any feeders, cells at all stages of hematopoiesis decreased to very low levels. In contrast, maintenance of LTC-IC was found to be supported by control murine stromal cells as effectively as by standard human marrow adherent layers. The presence of granulocyte colony-stimulating factor (G-CSF) and interleukin-3-producing M2–10B4 cells in combination was able to further enhance the maintenance and early differentiation of these cells without a decline in their proliferative potential as measured by the clonogenic output per LTC-IC. However, this effect was lost if granulocyte-macrophage CSF (GM-CSF)-producing feeders were also present. On the other hand, in the presence of GM-CSF-producing feeders, the output of mature granulocytes and macrophages increased 20- fold. These findings show that it is possible to selectively improve the maintenance of very primitive human hematopoietic cells in vitro or their output of mature progeny by appropriate manipulation of the long- term marrow culture system. Further exploitation of this approach should facilitate investigation of the mechanisms operative within the human marrow microenvironment in vivo and the design of protocols for in vitro manipulation of human marrow for future therapeutic applications.