The use of antibodies or antibody fragments for targeting tumors (either for tumor imaging or as carriers for drugs or toxins), has encountered problems of clearance, and non-specific or inefficient binding in clinical trials. A novel approach, linking two antibody variable fragments (Fvs), with a short peptide to generate a continuous polypeptide chain, may be able to overcome some of these problems. Since these single chain antibody variable regions (scFvs), are transcribed from constructed 'genes', large-scale production in, for example, E. coli, should be straightforward.