Specific pathogen-free chickens (bursectomized and x-irradiated (SBx-X), thymectomized and x-irradiated, x-irradiated, as well as nontreated) were inoculated (in the right tibiometatarsal joint) at the 4th week after hatching with a synovitis-derived Mycoplasma synoviae strain. Differences were not observed in recovery rate of mycoplasmas from tissues among these groups. The SBx-X chickens which were killed at 2 and 4 weeks after the intraarticular inoculation or which died during this period and bacteremia and severe synovitis with edema and heterophil infiltration in synovial fluid and in area from synovial membrane to subcutaneous tissue. Swelling of left leg joints or foot pads were observed mainly in the SBx-X group. A significant difference in the mortality rate and severity of airsacculitis was observed between SBx-X and other groups. The thymectomized and x-irradiated chickens did not show edema, synovial fluid, or swelling in the joint area. However, small lymph follicles composed of lymphoid and plasma cell were formed locally in synovial membranes of various joints. These results indicated that bursa-dependent lymphocytes are correlated with resistance to develop the lesions and that thymus-dependent lymphocytes are necessary for macroscopic lesions to develop.