Highly purified murine granulocyte-macrophage progenitor cells (CFU-GM) were used as target cells to assess the possible direct effects of purified preparations of recombinant murine gamma-interferon, prostaglandin E, recombinant human heavy chain (acidic) ferritin, and recombinant human tumor necrosis factor alpha (TNF-alpha) on progenitor cells in vitro. Target CFU-GM, with cloning efficiencies of up to 84% and containing 0-3% morphologically recognizable accessory cells at the initiation of the culture period, were plated at a density of 100-150 cells/dish in the presence or absence of pure suppressor molecules. Colony formation was stimulated with either crude pokeweed mitogen-stimulated mouse spleen conditioned medium, pure natural murine macrophage colony-stimulating factor, or pure recombinant murine granulocyte-macrophage colony-stimulating factor. All four suppressor molecules were active in vitro against purified CFU-GM as assessed by their ability to inhibit colony or cluster formation. No apparent difference in the degree of responsiveness to prostaglandin E, gamma-interferon, or human heavy chain (acidic) ferritin was noted in the presence of pokeweed mitogen-stimulated mouse spleen conditioned medium, granulocyte-macrophage colony-stimulating factor, or macrophage colony-stimulating factor. In contrast, TNF-alpha in cultures containing macrophage colony-stimulating factor slightly, but significantly, potentiated colony formation. TNF-alpha also appeared more active at suppressing colony formation at lower concentrations in pokeweed mitogen-stimulated mouse spleen conditioned medium than in granulocyte-macrophage colony-stimulating factor-stimulated cultures of purified CFU-GM. The results suggest that TNF-alpha, human heavy chain (acidic) ferritin, gamma-interferon, and prostaglandin E can act directly at the progenitor cell level.