Despite their general applicability, conventional sandwich immunoassays suffer from the requirements of two antibodies recognizing different epitopes, and multiple long incubation/washing steps. In addition, the assay is not applicable to the measurement of low-molecular-weight haptens due to their size. Recently, a novel sandwich-type immunoassay that can overcome the limitations of conventional sandwich immunoassays was proposed. The assay, which utilizes antigen-dependent stabilization of an antibody variable region, requires less time and handling, and is particularly suitable for hapten measurement. The principle could be applied to several homogeneous assay formats, and the method has been shown to have superior sensitivity, as well as a lower time requirement compared to conventional assays.