The synthetic pentapeptide pGlu-Glu-Asp-Cys-Lys (SPI) was evaluated in vitro alone and in combination with recombinant human tumor necrosis factor-alpha (TNF-alpha) and/or interferon-gamma (IFN-gamma) for effects on colony formation by hematopoietic progenitor cells (HPC) present in low-density (LD), nonadherent low-density T-lymphocyte-depleted (NALT-), and highly enriched sorted progenitor cells from normal human bone marrow. Progenitor cells in NALT- fractions were further enriched by cell sorting using two-color fluorescence on a Coulter Epics 753 flow cytometry apparatus with My10 and HLA-DR monoclonal antibodies. The sorted My10 DR+ progenitor cell population had a cloning efficiency of up to 38% for granulocyte-macrophage colony-forming units (CFU-GM), erythroid burst-forming units (BFU-E), and multipotential colony-forming units (CFU-GEMM). SP1 inhibited, by up to 86%, each of the colony-forming cells in a dose-dependent fashion. The sensitivities of the different progenitor cells to inhibition by the pentapeptide were the same when My10 DR+ marrow cells were used, and the progenitor cells in the My10 DR+ fraction were more sensitive than the cells in the LD or NALT- fraction to inhibition by SP1. The suppressive activity of SP1 on purified HPC was confirmed when 10(-3) M SP1 completely inhibited colony and cluster formation from a population of mouse bone marrow cells in which one of two cells was a CFU-GM. The effects of SP1 were not absolutely cell-cycle-specific for human HPC, but the non-S-phase cells were less sensitive than the S-phase cells to the suppressive effects of SP1. SP1 synergized with TNF-alpha and/or IFN-gamma to inhibit proliferation of progenitor cells using both LD or My10 DR+ human marrow cells stimulated by recombinant human interleukin 3 (IL-3). These studies suggest that the suppressive effect of SP1 occurs in the absence of certain accessory cells (e.g., monocytes and T-lymphocytes), that this effect may be mediated directly at the level of the HPC, and that this pentapeptide can be considered a candidate modulatory molecule for HPC proliferation.