We are evaluating strategies for the inhibition of growth or the selective killing of tumor cells. Cell surface antigens which are exclusively expressed or which are enhanced in their expression in tumor cells might provide the means to target cytotoxic or cytostatic agents to these cells. Few tumor specific cell surface antigens have been found, but the enhanced expression of growth factor receptors has been described for several types of tumors. A prominent example is the overexpression of the c-erbB-2 receptor in a high percentage of primary breast and ovarian carcinomas. We have derived monoclonal antibodies against the extracellular domain of the c-erbB-2 receptor. The antibody molecules were genetically engineered to minimize their size and to allow for their functional modification. For this purpose the cDNA sequences corresponding to the variable domains of one monoclonal antibody (FRP5) were molecularly cloned and joined by a short linker. The resulting single chain antibody molecule (scFv) was expressed in bacteria and purified. We show in an immunoprecipitation experiment that this molecule retains its ability to recognize the c-erbB-2 extracellular domain. This molecule could become a valuable vehicle to specifically transport anti-tumor agents to breast cancer cells.