The c-erbB-2 proto-oncogene encodes a 185kDa protein p!85, which belongs to epidermal growth factor receptor family. Amplification of this gene has been shown to correlate with poor clinical prognosis for certain cancer patients. The monoclonal antibody A21 which directed against p185 specifically inhibits proliferation of tumor cells overexpressing p185, hence allows it to be a candidate for targeted therapy. In order to overcome several drawbacks of murine MAb, we cloned its VH and VL genes and constructed the single-chain Fv (scFv) through a peptide linker. The recombinant scFvA21 was expressed in Escherichia coli and purified by the affinity column. Subsequently it was characterized by ELISA, Western blot, cell immunohistochemistry and FACS. All these assays showed the binding activity to extracellular domain (ECD) of p!85. Based on those properties of scFvA21, we further constructed the scFv-Fc fusion molecule with a homodimer form and the recombinant product was expressed in mammalian cells. In a The c-erbB-2 proto-oncogene encodes a 185 kDa protein p! 85, which belongs to epidermal growth factor receptor family. Amplification of this gene has been shown to correlate with poor clinical prognosis for certain cancer patients. The monoclonal antibody A21 which directed hereinafter to overcome several drawbacks of murine MAb, we cloned its VH and VL genes and constructed the single-chain Fv (scFv) through a peptide linker. The recombinant scFvA21 was expressed in Escherichia coli and purified by the affinity column. It was been by ELISA, Western blot, cell immunohistochemistry and FACS. All these assays showed the binding activity to extracellular domain (ECD) of p! 85. Based on those properties of scFvA21, we further constructed the scFv-Fc fusion molecule with a homodimer form and the recombinant product was expressed in mamm alian cells. In a