Erythropoietin (Epo) response and binding was assessed in purified murine CFU-E and their descendants. Several features emerged. First, Epo on CFU-E is in rapid flux: Half-time for 125I-Epo internalization is approximately four to five minutes. Second, computer-aided Scatchard analyses indicate that greater than 70 high-affinity Epo-receptor sites on anemic animal CFU-E are sometimes already occupied by Epo acquired in vivo. When this is removed, 40% of greater than or equal to 370 sites per CFU-E belong to a high-affinity class (dissociation constant, kd: 73 pmol/L +/- 15 [SE]) and 60% belong to a low-affinity class (kd: 813 pmol/L +/- 246). Third, the few small colonies that develop from CFU-E in the absence of Epo are shown, by serial assay of 59Fe-heme biosynthesis, to stem from contaminating erythroblasts: a result consistent with our finding that, after eight-hour CFU-E culture, most erythroblasts no longer require appreciable Epo for growth. Thus, although the early need for Epo by CFU-E is nearly absolute, this need is not met by the often substantial Epo already on board. The inference is that repeated occupancy of the rapidly turning over Epo receptors is required. Fourth, Epo bound and/or internalized by CFU-E descendants decreases to 40% of zero-time levels after 14 hours in Epo-supplemented culture and disappears after 28 hours. Scatchard analyses indicate that 73 pmol/L kd receptor sites become undetectable at seven to eight hours, whereas 813 pmol/L kd sites are undiminished and only one-third less by 16 hours. This apparent disappearance of high-affinity sites and persistence of low-affinity sites suggests that (a) at least two gene products mediate Epo binding, eg, two different receptor polypeptides or one receptor and one cofactor which modulates affinity; (b) high-affinity sites mediate the growth function of Epo during the first eight hours of culture; and (c) lingering low-affinity receptors may mediate some unrecognized Epo function. Fifth, the efficiency with which 106- and 91-Kd CFU-E membrane polypeptides can be cross-linked to 125I-Epo is two- to threefold higher for cells labeled at high Epo concentrations than at low ones, which suggests that these polypeptides largely reflect low-affinity site reactions.