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The development of a rapid and specific assay for 17β-estradiol (E2) will accelerate its in vitro diagnostics and/or environmental pollution control. Here, we employed an open-sandwich (OS) selection scheme to improve the sensitivity for E2 in an OS immunoassay, which is based on antigen-dependent stabilization of the antibody (Ab) variable region, Fv, where the two domains (V(H) and V(L)) dissociated in the absence of an antigen. The V(H) domain of a cloned anti-E2 antibody displayed on M13 phage was randomly mutated, and after three OS biopanning rounds, a mutant that showed higher sensitivity in OS-ELISA for E2 was identified. Interestingly, compared with the wild-type V(H), the cross-reactivity of the mutant was significantly decreased for the analogous steroid testosterone, both in OS and competitive ELISAs. This is the first report concerning selection for an anti-hapten Ab without using any hapten-carrier conjugates, and the method will be especially suitable for selecting Ab fragments that show better performance in hapten OS immunoassays.