Mature hematopoietic cells of all lineages are derived from committed progenitor cells, which in turn are thought to originate from pluripotential stem cells. On the basis of in vitro clonogenic assay systems, different stages within this progenitor or stem cell compartment have been identified. Using cell separation techniques, it has been shown that all in vitro clonogenic cells are included in the minor population of bone marrow or peripheral blood cells expressing the CD34 molecule. CD34 positive cells have further been subdivided into subsets of clonogenic cells by their coexpression of certain characteristic molecules. Based on these immunophenotypic features, it has been shown that most immature multipotential clonogenic cells may be separated from the majority of already lineage committed colony forming cells. Additionally, within the committed progenitor compartment, molecules have been described allowing the specific identification of the earliest stages of granulomonocytic, erythroid or lymphoid cell differentiation. Apart from these lineage or differentiation stage specific molecules, CD34+ cells express a number of molecules that are supposed to function as cytoadhesion structures or cytokine receptors enabling these cells to interact with their stroma microenvironment.