Both transforming growth factor beta (TGF beta) and macrophage inflammatory protein 1 alpha (MIP-1 alpha) have been shown to be multifunctional regulators of hematopoiesis that can either inhibit or enhance the growth of hematopoietic progenitor cells (HPC). We report here the spectrum of activities of these two cytokines on different hematopoietic progenitor and stem cell populations, and whether these effects are direct or indirect. MIP-1 alpha enhances interleukin-3 (IL- 3)/and granulocyte-macrophage colony-stimulating factor (GM- CSF)/induced colony formation of normal bone marrow progenitor cells (BMC) and lineage-negative (Lin-) progenitors, but has no effect on G- CSF or CSF-1/induced colony formation. Similarly, TGF beta enhances GM- CSF/induced colony formation of normal BMC and Lin- progenitors. In contrast, TGF beta inhibits IL-3/ and CSF-1/induced colony formation of Lin- progenitors. The effects of MIP-1 alpha and TGF beta on the growth of Lin- progenitors were direct and correlate with colony formation in soft agar. Separation of the Lin- cells into Thy-1 and Thy-1lo subsets showed that the growth of Thy-1lo Lin- cells is directly inhibited by MIP-1 alpha and TGF beta regardless of the cytokine used to stimulate growth (IL-3), GM-CSF, or CSF-1). In contrast, two other stem cell populations (0% to 15% Hoechst 33342/Rhodamine 123 [Ho/Rh123] and Lin- Sca-1+ cells) were markedly inhibited by TGF beta and unaffected by MIP- 1 alpha. Furthermore, MIP-1 alpha has no effect on high proliferative potential colony-forming cells 1 or 2 (HPP-CFC/1 or /2) colony formation in vitro, whereas TGF beta inhibits both HPP-CFC/1 and HPP- CFC/2. Thus, MIP-1 alpha and TGF beta are direct bidirectional regulators of HPC growth, whose effects are dependent on other growth factors present as well as the maturational state of the HPC assayed. The spectrum of their inhibitory and enhancing activities shows overlapping yet distinct effects.