Twenty-seven hybrid cell lines producing monoclonal antibodies to Chlamydia psittaci were prepared by fusion of the mouse myeloma cells with spleen lymphocytes of mice immunized with C.psittaci pigeon or budgerigar strain. Reactions of these monoclonal antibodies were examined with five strains of C.psittaci and two of C.trachomatis serovars L2 and J by enzyme-linked immunosorbent assay, the microimmunofluorescent test and the complement fixation test. According to the reaction patterns to eight chlamydial strains, fifteen antibodies to a pigeon strain, P-1041, were divided into three groups: three genus-specific, eight subspecies-specific and four strain-specific antibodies. Twelve antibodies to a budgerigar strain, Izawa, were also tested to eight chlamydial strains and divided into two groups: two genus-specific and ten subspecies-specific antibodies. Physicochemical properties of the antigens recognized by different monoclonal antibodies were examined by KIO₄, pronase and heat treatments and by the immuno-blotting technique. Most genus-specific antibodies recognized the KIO₄-sensitive antigen, which was a pronase-resistant and heat-stable component of about 10, 000 daltons. Subspecies- and strain-specific monoclonal antibodies recognized the antigens, which were pronase-sensitive and heat-labile components of about 40, O00 or 90, O0O daltons.