Immunotoxins composed of cell-selective ligands covalently linked to peptide toxins have been developed for the treatment of chemotherapy relapsed or refractory malignancies including chronic lymphocytic leukemia (CLL). A number of CLL immunotoxins have been clinically tested including T101-ricin A, H65-ricin A, DAB(486)IL2, DAB(389)IL2, RFB4 (dsFv)-PE38 and anti-Tac(Fv)-PE38. Remissions have occurred in some patients without significant myelosuppression, but novel agents continue to be needed. Kay and co-workers in this issue of Leukemia Research have targeted interleukin-4 receptors (IL4R) on CLL B cells with a recombinant IL4 pseudomonas exotoxin fusion protein (IL-4(38--37)-PE38KDEL). A fraction of patients (19%) had CLL cells that were extremely sensitive to the immunotoxin. This novel agent may provide an important new therapeutic for use in the treatment of CLL.