A number of cytokines have been implicated in the suppression of myeloid stem and progenitor cell proliferation. It has been suggested that some of these act directly on the stem/progenitors themselves, based on the effects of these cells, plated in culture at low seeding densities, on highly enriched populations. These studies, however, do not definitively rule out effects on accessory cells. To more rigorously evaluate direct-acting suppressive effects of cytokines, such cytokines were assessed for their effects on colony formation initiated by single bone marrow (BM) or umbilical cord blood (CB) CD34 cells sorted into single wells in the presence of a combination of growth-stimulating cytokines (erythropoietin [Epo], steel factor [SLF], granulocyte-macrophage colony-stimulating factor [GM-CSF], and interleukin-3 [IL-3]) and in the presence or absence of serum. Under these conditions, it was demonstrated that H-ferritin, transforming growth factor-beta 1 (TGF-beta 1), and members of the chemokine family (macrophage inflammatory protein-1 alpha [MIP-1 alpha], MIP-2 beta, platelet factor 4 [PF4], IL-8, and macrophage chemotactic and activating factor [MCAF]) had direct significant suppressive activities on single stem/progenitor cells from adult human BM in the presence or absence of serum. Single sorted CB cells were much less sensitive to inhibition by these cytokines. The reasons for this differential sensitivity are not known. Of possible relevance to this for cytokines, such as H-ferritin and the chemokines that have actions during S-phase of the cell cycle, CB progenitors were in slower cycle at initiation of culture than were BM progenitors.