Iron uptake from transferrin (Tf) by Friend erythroleukemia cells was studied before and after induction of the cells by dimethyl sulfoxide (DMSO) in culture. The increase in iron uptake after 5 days of induction was not accompanied by a proportional increase in the number of transferrin receptors detected by 125I-labeled Tf binding, resulting in DMSO induced cells taking up iron with the rate of about 26 iron atoms per receptor per hour, compared to 15 atoms in uninduced cells. In agreement with this finding are results of the study of cellular 125I or 59Fe-labeled Tf kinetics. In the induced cells Tf endocytosis and release proceeded with identical rates and all the endocytosed iron was retained inside the cell. Tf release by uninduced cells was slower and a part of internalized 59Fe was released. Hemin inhibits Tf-bound iron uptake by DMSO induced cells but not by the uninduced cells. Correspondingly, addition of hemin had little effect on Tf kinetics in the uninduced cells, while the release of Tf was slower in hemin-treated DMSO induced cells. Different efficiency of iron release from internalized Tf, accompanied by changes in cellular Tf kinetics is therefore proposed as a factor determining the rate of iron uptake by developing erythroid cells.