A project has been initiated to produce by genetic engineering, a single polypeptide chain with the antigen binding ability of a bispecific antibody. We have produced several cloned plasmids containing inserts encoding single chain Fv (sFv) molecules. One of these, an anti-DNP sFv, has been expressed in E. coli as inclusion bodies. This protein has been refolded into a molecule with antigen-binding activity.