Monoclonal antibodies were established for antigenic analysis of feline and bovine Chlamydia psittaci. The monoclonal antibodies recognized lipoplysaccharide (LPS), 56-64, 84 or 86 kDa antigens. At least 5 antibody-binding sites were detected on LPS with the monoclonal antibodies. The 56-64 kDa antigen was suggested to have both polypeptide and carbohydrate antibody binding sites. Immunoblotting analysis of cat and cattle sera indicated that the 56-64 kDa antigen is an important antigen in host immune response. The monoclonal antibodies are extremely useful tools to analyse the structure and function of chlamydial antigens.