We have previously reported that 20 hours' preincubation of human bone marrow cells with interleukin-1 beta (IL-1) can protect early progenitor cells from 4-hydroperoxycyclophosphamide (4-HC) cytotoxicity. Since tumor necrosis factor-alpha (TNF alpha) shares many of the biologic properties of IL-1, we have compared the protective effects of TNF alpha with IL-1 against 4-HC. Incubation of human bone marrow mononuclear cells or an enriched progenitor population for 20 hours with either TNF alpha or IL-1 resulted in the survival of an increased number of single- and mixed-lineage colonies, including replatable blast cell colonies, while only rare colonies were seen in the control group. Antibodies to TNF alpha completely abolished the protection observed with IL-1, while antibodies to IL-1 alpha and IL-1 beta decreased but did not abolish the protection seen with TNF alpha. Combinations of low doses of TNF alpha and IL-1 showed synergy in their protective effects. Furthermore, no protection was observed by IL-1, IL- 1 bone-marrow-conditioned medium (IL-1-BMCM), or TNF alpha for HL-60, K562, KG1, KG1a, and DU.528 leukemic-cell lines or primary acute myelogenous leukemic (AML) blast cells from the lethal effects of 4-HC. In the case of HL-60 and KG1a cell lines, TNF alpha preincubation resulted in increased cytotoxicity. Furthermore, preincubation of a mixture of AML cells and normal bone-marrow cells with IL-1 + TNF alpha before 4-HC resulted in the protection of normal but not leukemic progenitors. These results suggest that TNF alpha is necessary for the protection of normal, early, human hematopoietic progenitors from 4-HC, while IL-1 is not mandatory but will synergize with TNF alpha to offer increased protection. In addition, no protection from 4-HC is observed by TNF alpha, IL-1, or IL-1-BMCM for primary leukemic blast cells or leukemic cell lines.