Rabbit reticulocytes were incubated with rabbit transferrin labelled with 59Fe and lZsI in media in which the NaCl was replaced by other electrolytes or sucrose. Iron and transferrin uptake by the cells was affected by changes in the pH, ionic strength, ionic composition, and the osmolarity of the medium. Uptake was maximal at pH 7.4. A reduction in ionic strength produced by replacing NaCl with sucrose inhibited the uptake in a concentration-dependant manner, greatest inhibition occurring at lowest salt concentration. Similar results were obtained when KC1, LiC1, RbC1, Na,SO,, or K,S04 were used instead of NaC1. Low ionic strength was found to inhibit the endocytotic uptake of transferrin labelled with colloidal gold, but had only a small effect on transferrin binding to cell membrane receptors. I t was concluded that low ionic strength inhibits iron uptake primarily by blocking the endocytosis of transferrin. Three salts, NH4C1, CaCl,, and MgCl,, produced different results from the above. NH,Cl inhibited iron uptake at all concentrations used. This action was due to an effect on the release of iron from transferrin, which appeared to be taken up by the cells in a normal manner. When the ionic strength of the sucrose medium was increased by adding low concentrations of CaCl,, iron uptake was greater than with equivalent concentrations of NaCl. However, with CaCI, concentrations above 10 mM, iron uptake was inhibited, due to inhibition of transferrin uptake, possibly by blocking endocytosis. By contrast, MgCl, stimulated iron uptake at all concentrations used. The results are discussed in terms of the possible effects of ionic strength, pH, and ionic composition of the extracellular fluid on the three main steps involved in iron uptake by immature erythroid cells: transferrin-receptor interaction, endocytosis, and iron release from transferrin.