3′‐azido‐3′‐deoxythymidine (AZT), the main antiviral drug used in AIDS treatment, is known to induce anemia and neutropenia. These effects have been attributed to its toxicity to hematopoietic progenitors. In this report, we present a new approach to reduce AZT hematotoxic‐ity by using an inhibitory factor of the hematopoietic stem cells, the tetrapeptide AcSerAspLysPro (AcSDKP, Seraspenide), which has been shown to increase the survival of mice subjected to high doses of chemotherapy and to block reversibly the cycling of human granulocyte‐macrophage colony forming unit (CFU‐GM) and burst forming unit erythroid (BFU‐E) progenitors. Normal bone marrow mononuclear cells (BMMNC) from 14 subjects were incubated with or without AcSDKP (10−10 M) for 20 h and with or without AZT (100 μM) for another 2 h. After washing, cells were plated in methylcellulose in the presence of interleukin 3 (IL‐3), granulocyte‐macrophage colony stimulating factor (GM‐CSF) and erythropoietin (EPO). Under these conditions, the preincubation of cells with AcSDKP reduced significantly the toxicity of AZT to both BFU‐E and CFU‐GM at least in 3 out of 8 and 4 out of 10 cases, respectively. A careful statistical analysis of these observations indicates that AcSDKP may be an efficient factor in preserving progenitors against AZT‐induced hematopoietic toxicity.