Monoclonal antibody (mAb) CC49, a murine IgG1, reacts with the tumor-associated glycoprotein-72 expressed on a variety of carcinomas. In clinical trials, radiolabeled CC49 has shown excellent tumor localization to a variety of carcinomas. To minimize the immunogenicity of CC49 mAb in patients, a humanized CC49 (HuCC49) was generated by complementarity-determining region (CDR) grafting. The relative affinity of HuCC49 was 2–3-fold less than that of the murine mAb. With the aim of improving tumor targeting, attempts have been made to enhance the avidity of the HuCC49 mAb. Previous research has yielded a single geneencoded immunoglobulin, SCIgcCC49ΔCH1, which is a dimer of a single chain consisting of CC49 single-chain Fv linked to the NH2 terminus of the human γ1 Fc through the hinge region. This molecule is comparable to the mouse-human chimeric CC49 in terms of in vitro antigen binding properties, cytolytic activity, and rate of plasma clearance in athymic mice bearing human tumor xenografts. Recently, a single gene encoding a single-chain immunoglobulin consisting of a HuCC49 diabody attached to human γ1 Fc via the hinge region was constructed. The diabody, a bivalent antigen-binding structure, is made up of variable heavy (VH)/variable light (VL) domains and VL/VH domains. In each of the variable domain pairs, the VH and VL domains are linked through a short linker peptide. Meanwhile, the two pairs are linked via a 30-residue Gly-Ser linker peptide to yield two antigen-binding sites by lateral and noncovalent association of the VL of one pair with the VH of the other. Transfectomas expressing the single-gene immunoglobulin secrete a homodimer of about Mr 160,000 that reacts to tumor-associated glycoprotein-72. This tetravalent humanized antitumor immunoglobulin molecule may potentially be an efficacious therapeutic and diagnostic reagent against a wide range of human carcinomas.