Data are presented indicating that the growth of 5 out of 5 murine lymphoid tumors can be inhibited in a synergistic fashion in vitro by combined treatment with the iron chelator deferoxamine (DFO) and an immunoglobulin G (IgG) monoclonal anti-transferrin receptor antibody (ATRA). A two-way dose/response analysis shows that the ATRA becomes more efficient as an inhibitor with increasing doses of DFO. Flow cytometric studies further support the view that IgG ATRAS impair transferrin receptor (TR) function by causing TR down-modulation and degradation, even when the presence of DFO acts to promote increased cell surface TR expression. It is also shown that an IgG ATRA is nearly as effective as an IgM ATRA in inhibiting tumor cell growth when used in combination with DFO. Finally, studies with the iron chelator picolinic acid show that it produces only additive, or very slightly supra-additive, effects when used in combination with the ATRA. Therefore, these studies not only continue to suggest that combination chelator/ATRA therapy warrants further investigation as a tool in the therapy of hematopoietic malignancies, but also make the following new points: (1) the clinically familiar iron chelator deferoxamine, but not all iron chelators, produces synergistic inhibition of tumor growth in vitro with ATRAS; and (2) IgG ATRAS, which may be clinically more attractive reagents than IgA or IgM ATRAS because of better access to extra vascular tissue spaces, have unexpectedly been found to function as powerful growth inhibitors when used in combination with DFO.