Transferrin, the major iron-binding protein in the plasma of vertebrate species, is an essential growth factor for cells in serum free medium. We have established a cell line, Fr, from peripheral blood mononuclear cells of a patient affected by Sézary syndrome. Fr cells show a very immature antigenic phenotype, while constitutively bearing transferrin receptor on their surface. Furthermore the Fr line does not produce or respond to interleukin 2. Finally its conditioned medium contains both a growth stimulating activity for the Fr cell line and a factor which inhibits T-lymphocyte proliferation. We have identified a protein, produced in large amounts by Fr cells, which shares the immunological properties of human transferrin. Our data suggest that this transferrin-like factor can act as an autocrine growth factor for the producer cells and as an inhibitory factor for normal lymphocytes.