Induction of erythroid burst formation from day-12.5 fetal liver BFU-Es by gp55-P require the presence of both SF and SCM. Fetal liver cells were infected with retroviruses encoding gp55-P or, as control, β-galactosidase and then plated in methylcellulose containing the indicated growth factors. Data represent the average of two typical experiments, each performed in duplicate; the variation was less than 20%. Cells plated solely in Epo without SF and SCM did not produce any BFU-E colonies.