A high percentage of human breast and ovarian tumors display amplified c-erbB-2 gene copies, leading to overexpression of the growth factor receptor. Its membrane location and elevated expression make the erbB-2 protein an appropriate target for a directed tumor therapy. We have used recombinant DNA technology to produce a single-chain antibody-exotoxin A (scFv-ETA) fusion protein which specifically binds the human erbB-2 receptor. The scFv portion is composed of the heavy- and light-chain variable domains of a monoclonal antibody which recognizes the extracellular domain of the human erbB-2 receptor. The bacterially produced scFv-ETA protein was shown to bind specifically to cells expressing the human erbB-2 protein. The scFv-ETA inhibits protein synthesis in erbB-2-expressing tumor cells at doses ranging from 2 to 200 ng/ml and is cytotoxic for these cells at equivalent doses. In athymic nude mice, administration of the scFv-ETA inhibited the growth of erbB-2-overexpressing human ovarian carcinoma cells.