Numerous factors that can influence the proliferation and differentiation in vitro of cells at various stages of hematopoiesis have been identified, but the mechanisms used by stromal cells to regulate the cycling status of the most primitive human hematopoietic cells are still poorly understood. Previous studies of long-term cultures (LTC) of human marrow have suggested that cytokine-induced variations in stromal cell production of one or more stimulators and inhibitors of hematopoiesis may be important. To identify the specific regulators involved, we performed Northern analyses on RNA extracted from human marrow LTC adherent layers, or stromal cell types derived from or related to those present in the adherent layer. These analyses showed marked increases in interleukin-1 beta (IL-1 beta), IL-6, and granulocyte colony-stimulating factor (G-CSF) mRNA levels within 8 hours after treatments that lead to the activation within 2 days of primitive hematopoietic progenitors in such cultures. Increases in granulocyte-macrophage (GM)-CSF and M-CSF mRNA were also sometimes seen. Bioassays using cell lines responsive to G-CSF, GM-CSF, and IL-6 showed significant elevation in growth factor levels 24 hours after IL- 1 beta stimulation. Neither IL-3 nor IL-4 mRNA was detectable at any time. In contrast, transforming growth factor-beta (TGF-beta) mRNA and nanogram levels of TGF-beta bioactivity in the medium were detected at all times in established LTC, and these levels were not consistently altered by any of the manipulations that stimulated hematopoietic growth factor production and primitive progenitor cycling. We also found that addition of anti-TGF-beta antibody could prolong or reactivate primitive progenitor proliferation when added to previously stimulated or quiescent cultures, respectively. Together, these results indicate a dominant negative regulatory role of endogenously produced TGF-beta in unperturbed LTC, with activation of primitive hematopoietic cells being achieved by mechanisms that stimulate stromal cells to produce G-CSF, GM-CSF, and IL-6. Given the similarities between the LTC system and the marrow microenvironment, it seems likely that the control of human stem cell activation in vivo may involve similar variations in the production of these factors by stromal cells.