Iron-loaded transferrin has been shown to be necessary for the support of cell proliferation in culture. This function depends upon interaction of transferrin with a specific high-affinity cell surface receptor. The present report is directed toward determining the consequences of the interaction of transferrin with this receptor on Concanavalin A-stimulated rat lymphocytes. Three specific questions have been posed: a) Is transferrin endocytosed following binding to its specific receptor in a temperature-dependent fashion? b) Following endocytosis, is the carrier protein released from the cell in a structurally and functionally intact form? and c) Is the cell surface transferrin receptor also endocytosed following ligand binding? The results provide affirmative answers to all questions. Using two independent probes of the cell surface versus intracellular location of transferrin we observed that cell-bound transferrin moved from the cell surface to the inside of the cell and subsequently back to the medium. This process occurred in a temperature-dependent fashion. When cells containing only intracellular transferrin were further incubated at 37 degrees C approximately 80% of cell-bound transferrin was released to the medium. Nearly all of this material retained reactivity with antibody to transferrin. In addition, endocytosed transferrin exhibited qualitatively and quantitatively equivalent binding reactivity with the transferrin receptor and showed identical electrophoretic mobility on SDS gel electrophoresis. Finally, using similar methodology to that employed with transferrin itself, we provide evidence that the specific receptor is also endocytosed.