AIM: To construct and produce a recombinant bispecific humanized single-chain Fv (sFv) /Interleukin-2 (IL-2)fusion protein by using mammalian cells.METHODS: The sFv/IL-2 protein was genetically engineered, and transfected to mammalian cells to determine whether the mammalian protein folding machinery can produce and secrete active sFv/IL-2 with high efficiency.RESULTS: The fusion protein was constructed and high efficiently expressed with yields up to 102 ± 4.2 mg/L in culture supernatant of the stably transfected 293cell line. This recombinant fusion protein consisted of humanized variable heavy (VH) and light (VL) domains of monoclonal antibody (mAb) 520C9 directed against the human HER-2/neu (c-erbB2) proto-oncogene product p185, and human IL-2 connected by polypeptide linker. The fusion protein was shown to retain the immunostimulatory activities of IL-2 as measured by IL-2-dependent cell proliferation and cytotoxicity assays.In addition to its IL-2 activities, this fusion protein also possessed antigen-binding specificity against p185, as determined by indirect ELISA using p185 positive SKOV 3ip1 cells.CONCLUSION: The large-scale preparation of the recombinant humanized sFv antibody/IL-2 fusion protein is performed with 293 cells. The recombinant humanized sFv antibody/IL-2 fusion protein may provide an effective meansof targeting therapeutic doses of IL-2 to p185 positive tumors without increasing systemic toxicity or immunogenicity.