When K562 human erythroleukemic cells are induced to differentiate by addition of hemin to their medium, the number of binding sites for transferrin on the cell surface is substantially reduced. This reflects an internalization of receptors since no such reduction is observed when the total binding sites in soluble extracts of uninduced and differentiating cells are compared. The internalization of transferrin receptors has also been shown using lactoperoxidase-mediated radioiodination of cell surfaces and by immune precipitation of total and surface labeled receptors using an anti-receptor monoclonal antibody. Transferrin receptors from uninduced and differentiating cells were partially purified by affinity chromatography on transferrin-Sepharose and shown to be disulfide-bridged homodimers of a polypeptide with an apparent molecular weight of approximately 90,000. This protein is a phosphoprotein that can be resolved by isoelectric focusing into three major and two minor forms. By digestion with bacterial alkaline phosphatase, it was shown that at least four of these forms are probably phosphorylation variants of a single polypeptide. As differentiation proceeds, the proportions of the individual forms of the receptor change with a shift to the more phosphorylated polypeptides.