Stimulation of progenitor growth is increased in cocultures of CD34⁺ cells with stroma. A total of 2,000 or 5,600 DR⁺ cells/well were plated in methylcellulose medium in transwell inserts placed in 6-well plates containing similar methylcellulose medium. Cultures were supplemented with 5 ng/mL IL-3 alone or with 25% SN from irradiated stroma grown in a separate plate (stroma conditioned medium; M2-10B4 stromal cell line). Equal numbers of cells were also plated in transwell inserts directly above an intact irradiated stromal feeder layer (M2-10B4 stromal cell line) in the bottom chamber in methylcellulose medium not supplemented with exogenous cytokines (stroma coculture). Cells were also plated directly in 24-well plates in methylcellulose medium supplemented with cytokines comparable with stromal SN (SN concentration cytokines), as described for Fig 6. Numbers within the figure indicate number of experiments. Comparison between stroma coculture and other conditions are as follows: ¥P < .005, ¥¥P < .001.