Expansion of hematopoietic progenitors under stimulation with SCF + TPO. After numbers of viable cells were serially counted, the cultured cells were incubated in methylcellulose culture supplemented with IL-3, GM-CSF, G-CSF, SCF, and EPO. After 14 days, GM colonies, erythroid bursts, and mixed erythroid colonies were scored. The data are the mean ± SD of the number of colonies per well in triplicate suspension cultures. The results shown are from one representative experiment of three. Similar results were obtained in the other two experiments. GM colonies (○), erythroid bursts (□), mixed erythroid colonies (▵), and total colonies (•).