Previous studies have shown that primitive human hematopoietic cells detectable as long-term culture-initiating cells (LTC-ICs) and colony-forming cells (CFCs) can be amplified when CD34⁺ CD38⁻ marrow cells are cultured for 10 days in serum-free medium containing flt3 ligand (FL), Steel factor (SF), interleukin (IL)-3, IL-6, and granulocyte colony-stimulating factor. We now show that the generation of these two cell types in such cultures is differentially affected at the single cell level by changes in the concentrations of these cytokines. Thus, maximal expansion of LTC-ICs (60-fold) was obtained in the presence of 30 times more FL, SF, IL-3, IL-6, and granulocyte colony-stimulating factor than could concomitantly stimulate the near-maximal (280-fold) amplification of CFCs. Furthermore, the reduced ability of suboptimal cytokine concentrations to support the production of LTC-ICs could be ascribed to a differential response of the stimulated cells since this was not accompanied by a change in the number of input CD34⁺ CD38⁻ cells that proliferated. Reduced LTC-IC amplification in the absence of a significant effect on CFC generation also occurred when the concentrations of FL and SF were decreased but the concentration of IL-3 was high (as compared with cultures containing high levels of all three cytokines). To our knowledge, these findings provide the first evidence suggesting that extrinsically acting cytokines can alter the self-renewal behavior of primary human hematopoietic stem cells independent of effects on their viability or proliferation.