A recombinant immunotoxin consisting of the Fv portion of the anti-CD22 antibody covalently fused to a 38 KDa fragment of Pseudomonas exotoxin-A (PE38) with potential antineoplastic activity. The Fv portion of anti-CD22 immunotoxin CAT-8015 binds to CD22, a cell surface receptor expressed on a variety of malignant B-cells, thereby delivering the toxin moiety PE38 directly to tumor cells. Once internalized, PE38 induces caspase-mediated apoptosis via a mechanism involving mitochondrial damage and blocks translational elongation by binding to elongation factor 2 (EF-2). Anti-CD22 immunotoxin CAT-8015 exhibits a greater affinity for CD22 than its predecessor, anti-CD22 immunotoxin CAT-3888 (BL22 immunotoxin), and hence may be more effective against tumor cells expressing lower levels of CD22.