Iron uptake and the transferrin endocytotic cycle were studied in Friend erythroleukemia 707 cells cultured over a period of 5 days after induction with dimethylsulphoxide, or without it. The increase in iron uptake observed 5 days following induction with dimethylsulphoxide is not associated with a corresponding increase in the number of transferrin receptors. While the uptake rate of induced cells is 26 atoms of iron per one transferrin receptor per hour, in non-induced cells, the rate is only 15 atoms of iron. Induced cells retain all iron, with endocytosis and transferrin release occurring at the same rate. By contrast, the rate of iron release from non-induced cells is slower and part of the iron bound to transferrin returns from the cell. Haem inhibits iron uptake in induced cells and has no effect on non-induced cells. Its regulatory role is apparently confined to the erythroid cell with massive haemoglobin synthesis. The differences in the efficacy of iron release from induced and non-induced erythroleukemia cells are, possibly, apart from the changes in the number of transferrin receptors, an additional factor involved in the control of cellular iron uptake.