Significantly more primitive human hematopoietic progenitors are maintained and differentiate when cultured separately from the stroma by a microporous membrane ("stroma-noncontact" cultures) than when cultured in direct contact with stromal layers ("stroma-contact" cultures). This suggests that diffusible stroma-derived factors may be sufficient for the in vitro induction of progenitor proliferation and differentiation. To further characterize stroma-derived factors that maintain long-term bone marrow culture initiating cells (LTBMC-IC), we compared the hematopoietic supportive capacity of human marrow stroma with that of the murine marrow stroma-derived fibroblast cell line M2-10B4 as well as two embryonic fibroblast cell lines, the human FHS-173-WE and the murine NIH-3T3 cell lines. We demonstrate that LTBMC-IC, present in human CD34+/HLA-DR- (DR- cells), are maintained equally well and give rise to similar numbers of committed progenitors, that is--colony-forming cells (CFC)--when cultured in contact with human marrow stroma or any cell line feeder (stroma-contact cultures). LTBMC-IC, cultured in marrow stroma or M2-10B4 stroma-noncontact cultures, were maintained significantly better and gave rise to significantly more CFC than when cultured in human marrow stroma or M2-10B4 contact cultures. However, LTBMC-IC maintenance and differentiation in FHS-173-WE or NIH-3T3 noncontact cultures was significantly less than in human marrow stroma or M2-10B4 noncontact cultures. These studies indicate that systematic comparison of diffusible growth stimulatory factors in conditioned media from M2-10B4 cells and FHS-173-WE may lead to the characterization of growth regulatory factors required for in vitro maintenance and differentiation of human primitive LTBMC-IC. Since diffusible factors from the M2-10B4 cell line can support human hematopoiesis, our observations may also have important implications for in vitro stem cell expansion protocols.