The 'art' genes encode specific arginine uptake proteins, and are repressed by the repressible promoters of ArgR, affecting transcription of artJ [1,2]. Cpb0502, the arginine-binding periplasmic protein 2 precursor from Chlamydophila pneumoniae TW-183 strains, is responsible for arginine transport. As C. pneumoniae is difficult to isolate and culture, there have been many studies of better ways to detect it. A microimmunofluorescence assay (MIF) is still considered to be the 'gold standard' for detecting C. pneumoniae. Although MIF has its own limitations, a number of immunogenic antigens have been shown to be C pneumoniae specific by this test. Here, we report Cpb0502 as a specific immunogenic antigen against C pneumoniae as it was detected only in human infection sera of C. pneumoniae but not in Legionella pneumophila and Mycoplasma pneumoniae infection sera, showing high specificity and sensitivity by MIF, western blot and ELISA analysis. And also the crystal structure of Cpb0502 was determined to be a dimer at 2.07 angstrom, revealing a similar backbone structure to a histidine kinase receptor, HK29S. Therefore we may suggest that Cpb0502 is a candidate immunogenic antigen for better diagnosis of C. pneumoniae. (C) 2012 Elsevier Inc. All rights reserved.