We investigated the effects of transforming growth factor-beta (TGF-beta) and macrophage inflammatory protein-1 alpha (MIP-1 alpha) on very primitive CD34++CD38- and on more mature CD34++CD38+ human hematopoietic progenitor cells by means of a two stage pre-colony-forming cell (pre-CFC) assay. The first (liquid) stage of this assay allows evaluation of the effects of TGF-beta and MIP-1 alpha on the "primary" proliferation of the progenitors under study and on the generation of "secondary" colony-forming cells (CFC, cells for which a second stage semisolid clonogenic assay was used as a read-out). TGF-beta inhibited the proliferation and CFC generation of CD34++CD38- and CD34+CD38+ cells, showing the strongest inhibitory activity on CD34++CD38- cells. MIP-1 alpha exerted a weaker inhibitory activity on CD34+2CD38- cells, whereas it enhanced the primary proliferation of CD34+CD38+ cells and generation of secondary CFC in this subpopulation. Thus, TGF-beta, and MIP-1 alpha both inhibit very primitive CD34+2)CD38- cells, but they are not equally potent. The effects of TGF-beta and MIP-1 alpha on more mature progenitor cells are more complex. Our results and data from the literature indicate that, as progenitor cells mature, they reach a "pivotal point" at a certain stage in their differentiation pathway, depending on the inhibitor, where they are no longer inhibited or where they may even be stimulated by the former inhibitor to proliferate.