The cDNAs of κ chain and Fd fragment of anti-gastric cancer mAb 3H11 were amplified by RT-PCR using degenerate primers for framework region 1(FR1) and cloned into an Fab expression vector. Expression of Fab could be detected but with no antigen binding activity. Then the Vκ and Fd genes were corrected to its genuine sequence by PCR mediated mutagenesis. The reconstructed Fab containing either corrected κ chain or Fd or both were expressed in E.coli at similar level. Correction of any one of the V region genes could partially resume the antigen binding activity. This result indicated that PCR primers introduced Vκ and Fd N terminal changes may seriously affect the antigen binding activity.