Cloned human stromal cell lines representing functionally distinct cellular components of the marrow microenvironment were generated to serve as tools for identifying gene products that regulate hematopoiesis. Oligonucleotide arrays, or "gene chips" were used to provide a comprehensive comparison of gene expression among the cell lines. One line, designated HS-5, was found to secrete large amounts of cytokines, and conditioned media from this line was found to support the ex vivo expansion of both immature and mature progenitors. In contrast, a second line, designated HS-27a, does not secrete known cytokines but does support cobblestone area formation by CD34+/38lo cells. HS-27a, but not HS-5, was also found to express hJagged1, a ligand for Notch1, which may function to influence cell fate decisions of hematopoietic precursors. Both cell lines are currently being used to identify other gene products that regulate hematopoiesis and to generate reagents that will allow more formal evaluation of the putative role of hJagged1 in hematopoietic cell fate decisions.