Progenitor growth is stimulated by the concentrations of cytokines present in stromal SN. From 2,000 to 4,000 CD34⁺ cells/well were plated in methylcellulose cultures supplemented with 5 ng/mL IL-3 alone, 5% conditioned medium from 5,637 cells, or with a combination of recombinant human cytokines in concentrations comparable with stromal SN (50 pg/mL G-CSF, 5 pg/mL GM-CSF, 35 pg/mL SCF, 15 pg/mL LIF, 250 pg/mL IL-6, and 75 pg/mL MIP-1α). All cultures were supplemented with 3 IU/mL recombinant EPO. The no-cytokine cultures were not supplemented with any cytokines other than EPO. BFU-E, CFU-MIX, and CFU-GM colonies were enumerated on day 14 and HPP-CFC on day 28. Numbers within the figure indicate number of experiments. Comparison between no cytokines and SN concentrations are as follows: **P < .01, ¥P < .005, ¥¥P< .001.