The murine monoclonal antibody 125E11 is an IgG which recognizes PreS 1(21-47) fragment of large hepatitis B surface antigen. It has been successfully used for clinical detection of HBV virion in serum of hepatitis B patients. In present study, the genes of variable region in heavy chain (VH) and light chain (VL) of 125E11 have been cloned. Sequence analysis of cloned VH gene and VL gene showed that they had general characterization of immunoglobin variable region genes. According to Kabat classification, VH gene and VL gene belong to VH10 family, subgroup HID and V-K family subgroup 1, respectively. An expression vector of 125E11 single-chain Fv antibody fusion protein, in which VH and VL peptide were connected by a flexible linker (Gly(4)Ser)(3), was constructed. The scFv fusion protein was highly expressed in Escherichia coli mainly in inclusion body form. Using urea and pH gradient gel filtration method, the refolding of scFv was efficiently achieved. The refolding efficiency reached about 11 % and 2.7 mg refolded scFv was obtained from 1 L of culture. The binding activity and specificity of 125E11 scFv against PreS1 (21-47)-containing antigen were also analyzed. (c) 2005 Elsevier Inc. All rights reserved.