UNLABELLED: The Western blot (WB) method was verified for serological diagnosis of chlamydial infections. MATERIAL AND METHODS: For testing, sera previously examined by the microimmunofluorescence (MIF) test with either ambiguous results or those suggesting persistent infection were used. RESULTS: Whereas the investigation confirmed adequate sensitivity and specificity of the MIF test for diagnosing Chlamydophila pneumoniae infection, it was less sensitive in case of Chlamydia trachomatis. Long-term persistence of IgA antibodies, detected by the MIF test, was often not confirmed by WB. CONCLUSIONS: The results support the view that detecting antibodies alone, without appropriate clinical symptoms, is not sufficient for antibiotic treatment of any infection.