A recombinant immunotoxin consisting of the single chain anti-mesothelin monoclonal antibody SS1(dsFv) linked to Pseudomonas exotoxin PE-38. The monoclonal antibody moiety of the agent binds to cells that express mesothelin, a cell surface glycoprotein which may be overexpressed in ovarian cancer, mesotheliomas, and some squamous cell carcinomas; after internalization, the exotoxin moiety inactivates eukaryotic translation elongation factor 2, thereby disrupting tumor cell protein synthesis.