We have constructed genes expressing single–chain antigen binding proteins (scFv) which recognize the human erbB–2 receptor. These genes encode the heavy and light chain variable domains of an erbB–2 receptor specific monoclonal antibody, MAb FRP5, connected by a peptide linker. In order to express a bifunctional molecule, a bacterial alkaline phosphatase gene was fused 3′ to the scFv gene. The scFv(FRP5) and scFv(FRP5)–alkaline phosphatase fusion protein (scFv(FRP5)–PhoA) expressed in E. coli specifically recognize the human erbB–2 protein and compete with MAb FRP5 for binding to the receptor. The bound scFv(FRP5)–PhoA protein can be detected directly on tumor cells using a substrate for alkaline phosphatase, showing that the chimeric protein retains both binding and enzymatic activity.