⁵⁹Fe-Tf and ⁵⁹Fe-PIH uptake by S, R1, and R2 cells. Cells were plated in medium containing equivalent amounts of ⁵⁹Fe (106 pmole Fe/mL) as either⁵⁹Fe-Tf or ⁵⁹Fe-PIH and incubated for 6 to 24 hours. (A) ⁵⁹Fe uptake by cells over 6 hours in complete medium; (B) ⁵⁹Fe uptake by cells over 24 hours in complete medium; (C) ⁵⁹Fe-PIH uptake over 24 hours in complete medium; (D) ⁵⁹Fe-PIH uptake over 24 hours in serum-free medium. Values shown represent means ± SE of an experiment performed in triplicate. Similar results were obtained in two additional experiments.