To find a new marker for sero-diagnosis of Chlamydia trachomatis infection, Western blot (WB) method was performed by using C. trachomatis infected HeLa229 cell extract (L2-ext) as antigen. Two series of sera of patients with pneumonia or cervicitis (4 sera each) were used for investigation and analysis of a band which fades earlier than others depending on weeks or antibody levels after therapeutic treatment. A 17 KDa band was found which tended to fade gradually, but did not completely disappear within the period of investigation. The band was also detected in sera of patients with cervicitis diagnosed by detection of C. trachomatis organisms (IDEIA-PCE Chlamydia (IDEIA) or Clearview test were positive at first visit to the clinic). Ten anti-C. pneumoniae antibody positive sera and five anti-Mycoplasma pneumoniae antibody positive sera were also tested as controls. The result was that a 17 KDa band was detected in 20 of 25 sera (80%) with IDEIA positive. 18 of 33 patients (54.5%) with Clearview positive-, and 12 of 16 (75%) sera with both tests positive. No band was found in the control sera. The frequency of antibody against 17 KDa antigen was almost completely identical with that obtained by microimmunofluorescence test and Peptide-Chlamydia-IgG test. These results show that a 17 KDa band may be used as a marker for detection of C. trachomatis antibody by the WB method. The antigen could be precipitated with salting out from the L2-ext with 60% saturation of ammonium sulfate of Hofmeister's method and it was digested with proteinase K. From the result of the amino acid sequence analysis, it was found that 17 KDa protein is the human nucleoside diphosphate kinase B.