Ba/F3 cells transfected with the full-length EpoR or the EpoR/β_IL3 chimera proliferate in response to Epo. Cells were washed free of IL-3 and incubated with indicated concentrations of Epo. After 40 hours, the cells were pulsed with MTT for a further 4 hours, and MTT reduction was measured as optical density at 550 nmol/L (OD₅₅₀) of solubilized cells. Data are plotted as the percentages of the maximal response of cells cultured in IL-3. Error bars represent SEM of triplicate samples. Similar results were obtained with several individual clones.