EGFRvIII is a ligand-independent, constitutively active variant of the epidermal growth factor receptor (EGFR) that is specifically expressed in gliomas and various other human malignancies and has been proposed as a target for directed tumor therapy. We have recently constructed a highly potent single-chain antibody-toxin, scFv(14E1)-ETA, which consists of the variable domains of the antibody 14E1 specific for human full-length EGFR genetically fused to a truncated form of Pseudomonas exotoxin A. We demonstrate here binding of 14E1 antibody to both full-length and variant EGFR. In contrast to a recombinant toxin containing transforming growth factor-β£ (TGF-β£) as a cell targeting domain, scFv(14E1)-ETA was highly active on cells expressing EGFR-vIII. Surprisingly, scFv(14E1)-ETA displayed cell killing activity on EGFRvIII-expressing cells that was up to 100-fold higher than on control cells expressing full-length EGFR. No differences in the binding affinities of scFv(14E1)-ETA to full-length EGFR or EGFRvIII were observed, suggesting that events downstream of immunotoxin binding are responsible for the increased sensitivity of EGFRvIII-expressing cells. This might have implications for the development of therapeutic reagents simultaneously targeting different forms of the EGFR.