Internalization of ⁵⁹Fe from membrane-associated ⁵⁹Fe-Tf and its incorporation into heme. After saturating the membrane TfR with ⁵⁹Fe-Tf (see Figure 1, the end of period 2), internalization was initiated by incubating cells at 37°C in the presence of 5-fold excess of ⁵⁶Fe-Tf. At the indicated times, samples were collected and washed, and membrane-bound ⁵⁹Fe-Tf was removed by incubating the cells with ⁵⁶Fe-Tf (Figure 1, period 4). Radioactivity in cells and that in heme was then evaluated. The values are displayed as the percentage of the original cell associated ⁵⁹Fe as it was before displacement by ⁵⁶Fe-Tf (Figure 1, beginning of period 4).