5-androstene-3β,17β-diol, a naturally occurring adrenal steroid, has been shown to protect mice exposed to γ ray total body irradiation (TBI), as apparent by enhanced survival, stimulated myelopoiesis and elevated nadirs of neutrophils and platelets. In the present study, rhesus monkeys (Macaca mulatta; n=8) were subjected to the midlethal dose of 6 Gy 6 MV TBI at a dose rate of 35 cGy/minute, after which 4 randomly selected monkeys (2 males and 2 females) received 15 mg/kg IM of the testing compound HE2100 for 5 consecutive days, starting at 2 hours after irradiation. Placebo monkeys (also 2 males, 2 females) received the carrier IM for 5 days. TBI resulted in profound pancytopenia in all monkeys, which lasted 3 to 4 weeks in the placebo control monkeys. However, treatment with HE2100 reduced the period of severe neutropenia, with an 8 days earlier neutrophil recovery to levels >0.5x109/L than the placebo group, i.e. 12.8 days (range 11–15) vs 20.8 days (range 17–22), respectively. The neutrophil recovery was also reflected by accelerated recovery of CD11b+ cells. CD16/56+NK cells trailed the neutrophils. Recovery of reticulocytes was markedly enhanced in the HE2100 group to reach levels of >1% in peripheral blood (PB) by day 18.8 (range 18–20), whereas placebo control monkeys did not reach this level until day 26.3 (range 23–32). Reticulocyte reconstitution was accompanied by a transient erythroblastosis in HE2100 monkeys, emphasizing the pronounced stimulation of red cell reconstitution. A most prominent effect of HE2100 was noted for platelet recovery, since HE2100 effectively counteracted the need for transfusions, with only 1 (range 0–2) transfusion needed to maintain platelets at the used high threshold level of >40x109/L as apposed to 4 (range 2–8) in the placebo controls. The reconstitution of PB cells was preceded by a markedly accelerated recovery of immature bone marrow (BM) cells in HE2100 treated monkeys. At 2 weeks post-TBI, numbers of BM CD34+ cells approached pre-irradiation levels, whereas placebo monkeys did not reach such levels before the 4th week after TBI. Consequently, CD34+ cells in BM of HE2100 monkeys differed as much as 2 logs from those in placebo controls at 2 weeks after TBI, which was also reflected in the clonogenic progenitor cells GM-CFU and BFU-E, in accelerated normalization of BM cellularity and in PB CD34+ levels. Direct local or systemic toxic effects were not observed during administration of the steroid, but HE2100 monkeys displayed an increase of, on average, 10% in body weight due to fluid retention starting in the 2nd week after TBI, resulting in transient edema, which resolved without sequelae. This preclinical study characterizes HE2100 as a most potent novel agent to promote stem cell reconstitution and multilineage myelopoiesis after cytoreductive radiation exposure, resulting in enhanced reticulocyte, neutrophil and platelet recovery and profoundly decreased needs for transfusions and antibiotic treatment. The mechanisms involved are subject of further analyses.