⁵⁹Fe-Tf uptake by S and R1 CCRF-CEM cells at different times of proliferation. CCRF-CEM cells were grown for 0 to 72 hours in fresh medium and then used for ⁵⁹Fe uptake studies. Cells were plated at 2 × 10⁵ cells/mL in complete medium containing ⁵⁹Fe-Tf (228 pmole⁵⁹Fe/mL), and ⁵⁹Fe uptake by cells was determined at the times shown. (A) ⁵⁹Fe uptake by confluent, 0/72-hour cells; (B) ⁵⁹Fe uptake by cells previously grown for 24 hours in fresh medium; and (C) ⁵⁹Fe uptake by cells previously grown for 48 hours in fresh medium. (•) S cells; (○) R cells. Values are means ± standard error (SE) of a representative experiment performed in triplicate.