DESPITE recent advances using conventional approaches (surgery, chemotherapy, and radiotherapy), most human cancers remain incurable. For those patients who have unresectable cancers at the time of diagnosis, the prognosis remains poor. It is clear that new therapeutic approaches are urgently needed. In the past decade, development of the hybridoma technique by Kohler and Milstein1and advances in protein chemistry have led to the development of immunotoxins, a new class of cytotoxic agents consisting of a protein toxin coupled to a monoclonal antibody or a growth factor.2-4These toxins are made by plants or bacteria and act by arresting protein synthesis; cell death rapidly follows. The plant toxins generally inactivate ribosomes by cleaving a specific sequence in the 60S rRNA. The bacterial toxins,Pseudomonasexotoxin A (PE) or diphtheria toxin (DT), ribosylate adenosine diphosphate residues, ribosylate and thereby inactivate elongation factor 2, which is necessary for protein synthesis. Several