A murine hybridoma has been obtained that produces a monoclonal antibody against the human transferrin receptor. In contrast to previously characterized monoclonal antibodies that recognize the transferrin receptor, this antibody, designated 42/6, blocks the binding of transferrin to its receptor and inhibits the growth of the human T leukemic cell line, CCRF-CEM, in vitro. Inhibition of cell growth was dose dependent, and as little as 2.5 jtg of purified antibody per ml had a detectable effect, even though transferrin was present in the tissue culture medium in large molar excess. Cells grown in the presence of antibody for 7 days accumulated in S phase of the cell cycle. The addition of iron to antibody-treated cultures in the form of ferric complexes or ferrous sulfate did not overcome the growth inhibitory effects of the anti-transferrin-receptor antibodies. This result suggests that either transferrin is the only means by which CCRF-CEM leukemic cells can be provided with sufficient iron in vitro or that other factors in addition to iron starvation are involved in the antibody-mediated growth inhibition. The inhibition of cell growth by 42/6 monoclonal antibody suggests that monoclonal antibodies against proliferation-associated cell surface antigens, such as the transferrin receptor, may be useful pharmacological reagents to modify-cell growth in vitro.Iron plays an important role in cell growth and metabolism (1). Key reactions in energy metabolism and DNA synthesis are catalyzed by iron-containing enzymes, and it has been known for several years that transferrin, the major serum iron-transport protein, is an obligatory growth factor for cells growing in vitro (2-11). More recently, it has become apparent that cell surface receptors for transferrin are not only found in abundance on maturing erythroid cells and placental membranes but are expressed on proliferating cells in vitro and in vivo (12-16).We and others have obtained monoclonal antibodies that react with the human cell surface receptor for transferrin and confirmed that the expression of transferrin receptors is regulated by the growth state ofthe cells (17)(18)(19)(20)(21)(22). Although the antitransferrin-receptor monoclonal antibody, which we obtained and designated B3/25, inhibits the growth of a human melanoma cell line in nude mice (23), it does not interfere with either transferrin binding or cell growth in vitro. In this paper, we report the identification ofanother murine monoclonal antibody against-the human transferrin receptor that blocks transferrin binding and show that this antibody blocks the growth of a human T leukemic cell line in vitro.MATERIALS AND METHODS Cell Lines. CCRF-CEM, a human thymus-derived (T) leukemic cell line (24), was grown in RPMI 1640 medium supplemented with 10% horse serumn. BW5147, a murine T lymphoma cell line (25) was grown in Dulbecco's modified Eagle's medium supplemented with 10% horse serum.Monoclonal Antibodies. Monoclonal antibodies B3/25 and T56/14 directed against the human transferrin ...