Immunotoxins are a relatively new class of cytotoxic agents consisting of a catalytic toxin linked to an appropriate targeting ligand. The ligand directs the toxin to the surface of a tumor cell, whereupon the toxin enters the cell and catalytically inactivates the ribosome, thus disrupting protein synthesis and effecting cell death. Monoclonal antibodies (or their fragments) have been most commonly used to carry chemically conjugated toxins to proteins or antigens overexposed on the tumor cell surface, but specific ligands for tumor cell surface receptors could also provide effective targeting. The receptor for epidermal growth factor (EGFR) is overexpressed primarily in poor prognosis breast cancers that do not respond well to traditional therapies. Because EGFR is frequently overexpressed in breast cancer tissue and is associated with a poor prognosis, it is an attractive target for antitumor therapy. DAB389EGF is an EGFR specific fusion toxin produced with recombinant DNA techniques consisting of sequences for the enzymatically active and membrane translocation domains of diphtheria toxin plus sequences for human epidermal growth factor. DAB389EGF is a potent, EGFR specific, cytotoxic agent which rapidly inhibits protein synthesis by a mechanism of action similar to that of diphtheria itself. Preclinical studies in the laboratory and in animals now suggest the feasibility of investigating such an agent in the targeted therapy of patients with human breast cancer.