We have developed a long‐term culture system using the murine bone marrow stromal cells MS‐5 to support the growth of progenitor B cells with CD34^–, CD10⁺, CD19⁺, and cytoplasmic μ chain (Cμ)‐negative surface phenotype from human CD34⁺ cells purified from umbilical cord blood (CB). When 10³ CB CD34⁺ cells/well were seeded on MS‐5 stromal cells at the beginning of culture in the absence of exogenously added cytokines, progenitor B cells first appeared after 14 days, and the maximal cell production was achieved during the 6th week of culture. Intriguingly, the addition of recombinant human stem cell factor (rhSCF) and granulocyte colony‐stimulating factor (rhG‐CSF), but not rhIL‐7, strikingly enhanced the growth of progenitor B cells from CB CD34⁺ population cultured on MS‐5 stromal cells. The culture of progenitor B cells could be maintained until the 6th week of culture when some cells were revealed to have a Cμ⁺ phenotype, and a small number of cells had immunoglobulin μ chain on their cell surface in the presence of both rhSCF and rhG‐CSF. When CD34⁺ cells were cultured physically separated from the stromal layer by membrane, supportive effects of MS‐5 stromal cells for the growth of progenitor B cells were not observed. These results suggest that the present culture system could generate progenitor B cells to proliferate from CB CD34⁺ cells, that some of these progenitor B cells could differentiate into immature B cells in conjunction with rhSCF and rhG‐CSF, and that a species‐cross‐reactive membrane‐bound factor(s), which stimulates early human B lymphopoiesis, may exist in MS‐5 stromal cells. Further studies are required to investigate the mechanism how rhG‐CSF acts on progenitor B cells to allow their proliferation and differentiation.