Summary:In The remarkable proliferation and cell renewal processes in the human hematopoietic system are believed to be supported by a small population of hematopoietic stem cells. 1,2 A major challenge in stem cell research is the establishment of culture systems that facilitate in vitro maintenance and augmentation of stem cell activity. This is of great impor- tance not only for hematopoietic stem cell transplantation but also for gene therapy. It is also an important step towards cellular and molecular understanding of the regulatory mechanisms that mediate the commitment vs self-renewal decision of stem cells.In the last two decades a number of hematopoietic cytokines have been identified. Studies using recombinant hematopoietic cytokines have revealed that hematopoietic cytokines alone or in combination support the proliferation of hematopoietic progenitors and that it is possible to increase the number of progenitors in culture. 3 Recently, several groups of investigators have reported that combinations of early-acting cytokines support the maintenance or expansion of progenitors and even of transplantable stem cells in both murine 4-7 and human systems. [8][9][10][11] On the other hand, however, the importance of stromal cells in the maintenance and expansion of primitive progenitors including transplantable stem cells have been repeatedly demonstrated. 12-20 A number of stromal cell lines have been reported to support the proliferation of human primitive progenitors. MS-5 is one such cell line, derived from noninfected Dexter-type murine long-term marrow culture, which supports CFU-S maintenance 21 and acts synergistically with human growth factors to stimulate the formation of blast colonies and macroscopic colonies from CD34 + CD38 Ϫ primitive progenitors in short-term methylcellulose assays. 22 MS-5, alone 23 or in combination with SCF and G-CSF, 24,25 also supports the proliferation of primitive lymphohematopoietic progenitors that are capable of differentiation along both myeloerythroid and lymphoid lineages and their differentiation toward the B cell lineage. Furthermore, a recent study has demonstrated that, in the presence of multiple cytokines, MS-5 cells promote a net increase of LTC-IC through self-renewal divisions. 26 In the present study, we demonstrate synergistic effects of MS-5 cells and early-acting hematopoietic growth factors, FL and TPO on the expansion of human cord blood CD34 + cells, CD34 + CD38 Ϫ cells, CFU-C and CAFC. Using an SRC assay we also found that transplantable stem cell activity was slightly augmented or at least maintained in our culture system.