Hematopoietic stem cells (HSCs) in the bone marrow differentiate into diverse blood cell lineages via several rounds of lineage-restricted commitment and expansion, followed by lineage-specific proliferation and maturation. Our laboratory has been investigating the regulation of HSC self-renewal and differentiation for more than 15 years. Information on bone marrow physiology can be exploited to modulate hematopoietic cell differentiation. For example, cells from the erythroid (E) and megakaryocytic (Mk) lineages move from regions of low pO2 and low pH towards the marrow blood vessels as they mature. Consistent with this process, we have shown that differentiation along the E and Mk lineages can be inhibited and even blocked at an intermediate stage by culturing cells at subvascular pO2 or pH, and can be dramatically accelerated by using supervascular pO2 and pH values. In a similar manner, recent information regarding the cellular composition of the stem cell niche has inspired the development of culture systems that mimic the in vivo presentation of ligands for HSC cytokine receptors and adhesion molecules. We characterize hematopoietic cell responses by measuring gene expression patterns using microarrays, signal transduction and protein expression using flow cytometry, and lineage-specific changes in cell-surface antigen expression, morphology, and functional activity. Results from these assays have suggested novel approaches to further modulate cell differentiation. I will discuss some of the lessons we have learned and highlight some of our recent results.