Tumor necrosis factor-alpha (TNF-alpha), a product of activated macrophages that is cytotoxic to tumor cells, could be used to purge tumor cells from bone marrow before autologous bone marrow transplantation for hematologic malignancies and/or solid tumors. To determine whether exposure to TNF-alpha would have an inhibitory effect on hematopoietic progenitors, we incubated normal human bone marrow with a wide range of concentrations of recombinant human TNF. In order to mimic the conditions that would be used in bone marrow purging, bone marrow cell suspensions were incubated with TNF in doses ranging from 500 to 100,000 U/ml for 24 hours, and were assayed for colony formation in agar. We noted a dose-dependent inhibition of total colony-forming units (CFU) at days 7 and 14, with 50% inhibition occurring at 60,000 U/ml of TNF. TNF exerted a differential effect on CFU so that colony formation by erythroid (CFU-E), multipotential (CFU-GEMM), and macrophage (CFU-M) progenitors was suppressed to a greater extent than that by granulocyte progenitors (CFU-G). However, even after preincubation with TNF at high doses such as 100,000 U/ml, the inhibitory effects of TNF could be abolished by washing cells before culturing. This study demonstrates that hematopoietic precursors survive treatment with TNF at doses that have been shown to be cytotoxic to tumor cells. Although TNF has a significant inhibitory effect on the growth of erythroid, multipotential, and macrophage progenitors in vitro, this effect depends on continuous exposure to TNF for more than 24 hours. Thus, TNF may be useful as a bone marrow purging agent against tumor cells, with relative sparing of normal marrow elements.