The precise regulation of epidermal growth factor receptor (EGFR) is crucial for its function in cellular growth control Although many antibodies against EGFR have been developed and used to analyze its regulation and function, it is not yet easy to analyze activated EGFR specifically Activated EGFR has been mainly detected by its phosphorylation state using anti-phospho-EGFR and anti-phosphotyrosine antibodies In the present study, we have established novel monoclonal antibodies which recognize the activated EGFR independently of its phosphorylation Our antibodies detected active state of EGFR in immunoprecipitation and immunofluorescence, by recognizing the epitopes which are exposed through the conformational change induced by ligand-binding Furthermore, we found that our antibodies preferentially detected the conformation of constitutively active EGFR mutants found in lung cancer cell lines These results indicate that our antibodies may become novel research and diagnostic tools for detecting and analyzing the conformation of active EGFR in various cells and tissues. (C) 2010 Elsevier Inc All rights reserved