BACKGROUND The IgG isotype of antibodies is very important for their biological functions such as complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC). To increase the biological activity of a novel human monoclonal antibody (C2-45) against carcinoembryonic antigen (CEA), we tried to genetically convert its isotype from IgG4 to IgG1. METHODS VH and VL genes were cloned from the parental antibody C2-45 (IgG4) and inserted into the pAc-kappa-CH3 expression vector which contained the constant region gene of human IgG1. The recombinant gene was transfected into Sf9 insect cells to produce recombinant protein. The resulting recombinant protein, designated C2-45 (cIgG1), in the culture medium was purified by affinity chromatography and characterized for its CEA binding activity and biological activity. RESULTS The converted C2-45 (cIgG1) retained the original antigen-binding activity and showed significantly higher CDC and ADCC activities against CEA-expressing tumor cells than did the original C2-45 (IgG4). CONCLUSIONS C2-45 (cIgG1) may be useful for antibody-based immunotherapy of human CEA-expressing tumors.