Stimulation of progenitor growth is increased by the higher levels of IL-6 and G-CSF present in stromal cocultures. A total of 4,000 CD34⁺ cells/well were plated in methylcellulose medium supplemented with no cytokines or with a combination of recombinant human cytokines in concentrations comparable with stromal SN (5 pg/mL GM-CSF, 35 pg/mL SCF, 15 pg/mL LIF, and 75 pg/mL MIP-1α) and IL-6 and G-CSF either at the concentrations present in stromal SN (250 pg/mL IL-6 and 50 pg/mL G-CSF) or at the concentrations present in cocultures of stroma with CD34⁺ cells (1,000 pg/mL IL-6 and 250 pg/mL G-CSF). All cultures were supplemented with 3 IU/mL recombinant EPO. Colonies were enumerated on day 14. N = 4. Comparison between SN concentration cytokines and SN concentration cytokines with higher concentrations of IL-6 and G-CSF: **P < .01.