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To clone the gene coding the immunodominant region in the chlamydial protease-like activity factor (CPAF) from Chlamydophila pneumoniae, to analyze immunocompetence of the expressed protein,and to evaluate its value in serodiagnosis, the CPAF immunodominant region gene was amplified, ligated into a pGEX6p-2 vector, and then the expressed recombinant protein was purified with glutathione Stransferase (GST) agarose gel FF after renaturation, then identified by SDS-PAGE and Weste blot. A new indirect ELISA was developed with the purified protein as coating antigen. The immunogenicity of the recombinant protein was evaluated by immunization to New Zealand rabbits, and its immunoreactivity was analyzed by reacting with anti-C, pneumoniae antibody. 300 clinical sera samples were respectively detected by microimmunofluorescence (MIF) as reference method and the indirect ELISA, and the difference between the two methods was analyzed. Cross-reactivity against Chlamydia trachomatis was investigated with the indirect ELISA to detect anti-C, trachomatis positive antisera. The results indicated that a 51.3 kDa recombinant protein was obtained. Weste blot assay proved that the recombinant protein could merely specifically react with human anti- C. pneumoniae antisera. The titers of the specific IgG antibodies in the immunized New Zealand rabbits were above 1 : 16 000. Anti- C. pneumoniae IgG positive and negative reference sera were detected with the indirect ELISA, and the concordance rate of negative and positive results were both 100% (40/40). The sensitivity and specificity of the indirect ELISA in comparison with MIF were 93.8% (45/48) and 100% (252/252) separately by detecting 300 clinical sera samples, and the concordance rate between the two methods was 99.0%. No cross reaction against C. trachomatis was found with the indirect ELISA to detect anti-C, trachomatis positive antisera. In conclusion, the prepared recombinant protein of the CPAF immunodominant region shows excellent immunocompetence and can be used to develop a new indirect ELISA as a method to detect anti-C, pneumoniae antibody for diagnosis of C. pneumoniae infection.