Effect of pretreatment with Epo and IL-3 on responsiveness to the differentiation inducing action of Epo. (Top panel) G418^r clones E-1, E-3, E-4, and E-10 in IL-3–containing medium (IL-3 0 hours) were washed and exposed to Epo for 12, 24, and 48 hours. Epo added to cells without washing (Epo + IL-3 2 days) was compared with untreated cultures (IL-3 2 days). (Bottom panel) The same clones in IL-3–containing medium (IL-3) washed and pretreated with Epo and IL-3 for 2 days (Epo + IL-3 2 days). Cells were then washed again and exposed to Epo for 12, 24, and 48 hours.