A recombinant immunotoxin consisting of the Fv portion of the anti-CD22 antibody RFB4 fused to a fragment of Pseudomonas exotoxin-A with potential antineoplastic activity. BL22 immunotoxin binds to CD22, an antigen expressed in B-cell malignancies, thereby delivering its toxin directly to tumor cells. The toxin moiety induces caspase-mediated apoptosis of tumor cells via a mechanism involving mitochondrial damage; it also blocks translational elongation via binding to elongation factor-2 in eukaryotic cells.