A bivalent fusion protein consisting of disulfide-stabilized Fv fragments of an anti-HER2 (erbB2) monoclonal antibody (e23) and a truncated version of the M(r) 38 fragment of Pseudomonas exotoxin that lacks the toxin's cell binding domain. ERB-38 immunotoxin binds specifically to cells that overexpress HER2, the antigenic target of the monoclonal antibody; the exotoxin portion of the immunotoxin then lyses the cells bound by the antibody portion. or A toxic substance linked to an antibody that attaches to tumor cells and kills them.