StemRegenin 1 (SR1) acts by antagonizing the aryl hydrocarbon receptor (AhR). StemRegenin 1 increases the number of CD34 cells after 5 to 7 days with an EC50 of ~120 nM. StemRegenin 1 inhibits photoaffinity ligand (PAL) binding (IC50=40 nM) These results support the conclusion that StemRegenin 1 -induced CD34 cell expansion is mediated through direct binding and inhibition of the AhR. An aryl hydrocarbon receptor antagonist, StemRegenin 1 (SR1), robustly promotes ex vivo expansion of human CD34 cells. StemRegenin 1 treatment accelerates the proliferation of CD34 cells and decreases the expression levels of VentX.