Amifostine (WR-2721, Ethyol) is a phosphorylated aminothiol that affords broad cytoprotection from the myelosuppressive effects of antineoplastics. To further characterize its hematopoietic activities, we investigated the effects of amifostine and its dephosphorylated metabolite, WR1065, on the in vitro growth of human bone marrow progenitors. Preincubation exposure to amifostine or WR1065 stimulated the growth of colony-forming units granulocyte, erythroid, macrophage, megakaryocyte (CFU-GEMM) and erythroid bursts (BFU-E) from bone marrow mononuclear cells in a dose-dependent fashion. Over the concentration range tested (0.1-1000 microM), pretreatment with the aminothiols enhanced formation of CFU-GEMM up to five-fold and BFU-E nine-fold, compared to a three-fold increase in myeloid colony recovery. In CD34+ selected cells, preincubation with amifostine increased formation of CFU-GEMM up to 38-fold and produced macroscopic colonies, exceeding colony number in cultures initiated with optimal concentrations of interleukin-1 (IL-1), IL-3, or kit ligand (KL). When compared with recombinant human cytokines, amifostine enhanced IL-1 and IL-3 induced colony formation, although its stimulatory effect was less than additive. In contrast, pretreatment with amifostine antagonized the stimulatory effects of KL, whereas synergy was observed with concurrent exposure. Ex vivo expansion studies showed that amifostine alone supported and augmented the production of myeloid progenitors in secondary cultures. Similarly, under cytokine-deficient conditions, amifostine promoted cell survival and delayed apoptosis as measured by nucleosome generation. These data indicate that amifostine is a novel multipotent hematopoietic stimulant that augments the formation and survival of bone marrow progenitors.