In this work, the Z-domain of protein A was autodisplayed on the surface of the Escherichia coli outer membrane as a fusion protein of AIDA-1, and then layered on a microplate in order to develop a hypersensitive immunoassay. By using microplates with controlled hydrophilicity the formation of outer membrane layer was carried out, and the driving force for the layer formation was proven to be a hydrophobic interactions. Through the orientation control of the immobilized antibodies by using autodisplayed Z-domains. the limit of detection (LOD) and the sensitivity of this new immunoassay were determined to have improved as much as 30-fold compared to the conventional ELISA. The applicability of the new immunoassay for medical diagnosis was demonstrated by the detection of C-reactive protein (CRP) which is known to be a biomarker protein for inflammatory diseases. (C) 2009 Elsevier Inc. All rights reserved.