Efforts to change the fate of human hematopoietic stem cells (HSCs) and progenitor cells (HPCs) in vitro have met with limited success. We hypothesized that previously utilized in vitro conditions might result in silencing of genes required for the maintenance of primitive HSCs/HPCs. DNA methylation and histone deacetylation are components of an epigenetic program that regulates gene expression. Using pharmacologic agents in vitro that might possibly interfere with DNA methylation and histone deacetyletion, we attempted to maintain and expand cells with phenotypic and functional characteristics of primitive HSCs/HPCs. Human marrow CD34+ cells were exposed to a cytokine cocktail favoring differentiation in combination with 5aza 2′deoxycytidine (5azaD) and trichostatin A (TSA), resulting in a significant expansion of a subset of CD34+ cells that possessed phenotypic properties as well as the proliferative potential characteristic of primitive HSCs/HPCs. In addition, 5azaD- end TSA-pretreated cells but not the CD34+ cells exposed to cytokines alone retained the ability to repopulate immunodeficient mice. Our findings demonstrate that 5azaD and TSA can be used to alter the fate of primitive HSCs/HPCs during in vitro culture.