The factors that induce proliferation of the human hematopoietic stem cell are ill defined. Further characterization of such growth factors will be needed to develop ex vivo culture systems that induce prolonged proliferation and expansion of human hematopoietic stem cells. Human or murine hematopoietic progenitors that can initiate and sustain long-term culture systems (LTC-IC) represent a population of very primitive hematopoietic progenitors. When cultured in direct contact with stromal layers, we and others have demonstrated that a fraction of such LTC-IC can be maintained. In addition, stroma-free long-term cultures supplemented with two to nine cytokines can induce proliferation and differentiation of immature human hematopoietic progenitors. However, 70-90% of primitive LTC-IC are lost after five weeks in such cultures. We describe a "stroma-non-contact" culture system, in which progenitors are cultured separated from stroma by a 0.4 micron microporous membrane which prevents cell stroma contact but allows free passage of diffusible factors. Primitive progenitors in such cultures can not only differentiate into committed progenitors but also are maintained to a greater extent than in Dexter cultures. We will discuss the relative contribution of 1) direct contact between hematopoietic progenitors and bone marrow stroma, 2) soluble stroma-derived factors and 3) previously characterized growth promoting and presumed growth inhibitory cytokines in the in vitro maintenance and potential expansion of LTC-IC.