The prostate-specific membrane antigen (PSMA) is abundantly expressed on prostate cancer epithelial cells and its expression correlates with tumor progression. Therefore, a specific immunotherapy against this antigen may be a novel therapeutic option for the management of prostate cancer. We generated an anti-PSMA single-chain antibody fragment (scFv), called D7, by phage display from the monoclonal antibody 3/F11. By C-terminal ligation of the toxic domain of Pseudomonas Exotoxin A (PE40) to the genes of D7, the immunotoxin D7-PE40 was generated. D7 and D7-PE40 specifically bound to PSMA transfectants and to the PSMA expressing prostate cancer cell line C4-2. In addition, D7-PE40 showed a high serum stability and induced a 50% reduction of viability (IC50) in C4-2 cells at a concentration of 140 pM. In vivo, D7-PE40 was well tolerated in SCID mice up to a single dose of 20 microg, whereas higher doses induced severe hepatotoxicity with deaths of the animals. Immunotoxin treatment of mice bearing C4-2 tumor xenografts caused a significant inhibition of tumor growth, whereas mice with PSMA-negative DU 145 tumors remained unaffected. Owing to its high and specific cytotoxicity and its capability to inhibit prostate tumor growth in vivo the immunotoxin D7-PE40 represents a promising candidate for the immunotherapy of prostate cancer.