Iron uptake by rabbit reticulocytes was inhibited by spermine in a concentration-dependent manner. Examination of the single-cycle endocytosis of 125I-transferrin showed that a graded reduction in the exocytosis of transferrin was related to increasing extracellular spermine concentrations. This reduction could affect the recycling of transferrin receptors and resulted in the loss of membrane binding sites in spermine-treated cells. As large vacuoles were observed in cells treated with spermine, the endotubular function of these cells was probably affected. Spermine also enhanced the binding affinity of transferrin to membrane receptors. The mechanism for this enhancement was not clear.