Analysis of the role of SCF and BMP4 in increasing the number and size of stress BFU-E colonies in vitro. Spleen cells from C57BL/6 mice were preincubated in media containing no factors, SCF, BMP4, or BMP4 + SCF for 24 hours as indicated. The cells were then washed and plated in methylcellulose media containing the indicated factors for 5 days. (A) Stress BFU-Es were scored, and (B) stress BFU-E colony diameter was measured. The results presented are the average of 2 independent experiments. Error bars indicate SD.