In vitro maintenance and proliferation of human hematopoietic stem cells is crucial for many clinical applications. Early hematopoietic cells express low levels of FLT-3 and c-kit receptors, as well as the interleukin-6 (IL-6) receptor signal transducing element, gp130, but do not express IL-6 receptor itself. Therefore, we have attempted to maintain human cord blood or bone marrow CD34⁺ cells ex vivo in serum-free cultures containing stem cell factor (SCF) and FLT-3 ligand (FL) alone or together with a new recombinant molecule of soluble IL-6 receptor fused to IL-6 (IL6RIL6 chimera). The effect of IL6RIL6 chimera on the proliferation and differentiation of CD34⁺ cells was compared with that of each chimera component added separately. The engraftment potential of in vitro-cultured cells was determined using our recently established functional in vivo assay for primitive human severe combined immunodeficiency (SCID)-repopulating cells (SRC). We report here that IL6RIL6 chimera induced significantly higher levels of progenitors and SRC compared with SCF + FL alone or together with IL-6 and soluble IL-6 receptor. IL6RIL6 chimera prolonged in vitro maintenance of SRC for up to 14 days. Stimulation of CD34⁺CD38^−/low enriched cells with IL6RIL6 chimera maintained the early CD34⁺CD38^−/lowcell subpopulation, which could be detected in vitro for up to 14 days. Moreover, IL6RIL6 chimera preferentially stimulated the growth of early CD34⁺38^−/low cells, resulting in significantly higher levels of progenitors compared with more mature CD34⁺38⁺ cells. Taken together, these findings demonstrate the importance of IL6RIL6 chimera in stimulating the proliferation of early CD34⁺· CD38⁻gp130⁺IL-6R⁻cells in vitro and extended maintenance of progenitors and SRC.