Scatchard plots of ¹²⁵I-rhEpo to Epo-R–expressing TEPC clones E1, E2, and E4. Cells (10⁶) were incubated with ¹²⁵I-rhEpo with or without a 100-fold excess of unlabeled rhEpo (20 pmol/L to 200 nmol/L) for 3 hours at 37°C in RPMI containing 25 mmol/L HEPES, 5% FCS, 0.1% NaN₃ . B/F means bound radioactivity against free radioactivity.