Endosomal motility is required for ⁵⁹Fe availability. The endosomal compartment of reticulocytes was loaded with ⁵⁹Fe₂-Tf by treating the cells sequentially with bafilomycin (60 nM; 30 minutes at 37°C) and ⁵⁹Fe₂-Tf (1 μM; 30 minutes at 37°C). Where indicated, increasing concentrations of W-7 were included in the reincubation step (60 minutes at 37°C). The incorporation of ⁵⁹Fe into heme is shown as the percentage of total cell-associated ⁵⁹Fe. 0′, before reincubation; 60′, 60 minutes reincubation (in the absence of W-7). Error bars represent standard deviations (n = 3).