The use of antibodies to target tumor antigens has had limited success, partially due to the large size of IgG molecules, difficulties in constructing smaller single chain Fv (scFv) antibody fragments, and immunogenicity of murine antibodies. These limitations can be overcome by selecting human scFv directly from non-immune or semi-synthetic phage antibody libraries; however, the affinities are typically too low for therapeutic application. For hapten antigens, higher-affinity scFv can be isolated from phage antibody libraries where the V_Hand V_Lgenes of a binding scFv are replaced with repertoires of V genes (chain shuffling). The applicability of this approach to protein binding scFv is unknown. For this work, chain shuffling was used to increase the affinity of a non-immune human scFv, which binds the glycoprotein tumor antigen c-erbB-2 with an affinity of 1.6 × 10⁻⁸M. The affinity of the parental scFv was increased sixfold (K_d=2.5 × 10⁻⁹M) by light-chain shuffling and fivefold (K_d=3.1 × 10⁻⁹M) by heavy-chain shuffling, values comparable to those for antibodies against the same antigen produced by hybridomas. When selections were performed on antigen immobilized on polystyrene, spontaneously dimerizing scFv were isolated, the best of which had only a slightly lowerK_dthan wild type (K_d=1.1 × 10⁻⁸M). These scFv dimerize on phage and are preferentially selected as a result of increased avidity. Compared to scFv which formed only monomer, dimerizing scFv had mutations located at the V_H– V_Linterface, suggesting that V_H– V_Lcomplementarity determines the extent of dimerization. Higher-affinity monomeric scFv were only obtained by selecting in solution using limiting concentrations of biotinylated antigen, followed by screening mutant scFv from bacterial periplasm byk_offin a BIAcore. Using the proper selection and screening conditions, protein binding human scFv with affinities comparable to murine hybridomas can be produced without immunization.