The addition of recombinant murine tumor necrosis factor alpha (rmTNF-alpha) to serum-free methylcellulose cultures inhibited macrophage colony formation stimulated by purified colony stimulating factor-1 (CSF-1), recombinant granulocyte-macrophage-CSF (rmGM-CSF), and recombinant interleukin 3 (rmIl-3). The concentration of rmTNF-alpha inhibiting colony formation by 50% (IC50) was between 2 and 20 ng/ml. Erythroid colony formation in cultures with erythropoietin (EPO) alone or EPO, rmIl-3, and rmGM-CSF in combination were reduced to a much lesser extent. In established long-term marrow cultures (LTMC), addition of 20 and 200 ng/ml of rmTNF-alpha resulted in release of cells from the adherent layer during the first week. Treatment of cultures with rmTNF-alpha for 4 consecutive weeks led to prolonged inhibition of cell production lasting up to 8 weeks after cessation of treatment. One day after addition of a low dose of TNF (2 ng/ml), "fat" cells were no longer observed in the adherent layer. Our results indicate that TNF inhibition of hemopoiesis occurs both at the progenitor cell and stromal cell levels.