Murine erythroid colony-forming units (CFU-E) representing successive cell generations in a six-generation long in vitro maturation sequence were tested for their response to erythropoietin (Epo) by measurement of Epo-exposure times necessary to stimulate heme biosynthesis. Generation I CFU-E, which produce mainly 32-cell erythroid colonies, were isolated in 82% average purity from spleens of thiamphenicol-treated anemic animals via differential centrifugation. Generation II CFU-E, which produce mainly 16-cell colonies, were similarly isolated in 51% average purity. Although both types of CFU-E had equivalent dose sensitivity to and affinity for Epo, generation II CFU-E responded to shorter pulses of Epo than did generation I. Correlations between DNA cell-cycle profiles and 59Fe-heme biosynthesis resulting from pulsed exposures established that appreciable Epo response only begins when CFU-E attain early S-phase of generation II. Because CFU-E did not require Epo or other serum factors to pass from generation I to II and because the onset of Epo responsiveness coincided with the beginning of DNA replication in generation II, we suppose that differentiation has reprogrammed one or more of the events associated with generation II S-phase in CFU-E and that these alterations allow Epo to act. Further comparisons between CFU-E from generation I and II may allow us to identify the alterations in question and the nature of their interaction with Epo.