Specific - pathogen - free chickens were inoculated in the right tibiometatarsal joint with a synovitis-derived Mycoplasma synoviae strain before or during dexamethasone treatment. Development of synovitis in chickens inoculated during the drug treatment was apparently delayed in comparison with development of synovitis in non-treated chickens. Severity of clinical synovitis in chickens inoculated before the drug was given was apparently less than that in chickens not treated or in chickens treated with dexamethasone. Histopathologic changes in the early stage of the infection (1 to 2 weeks) were not modified by dexamethasone treatment, although those changes in the succeeding stage (6 to 7 weeks) were greatly lessened. A relationship was observed between the dosage of dexamethasone and the severity of synovitis, as well as the kinds of cells that infiltrated into the joint lesions. Although serum antibody titers in chickens treated with an excessive dose of dexamethasone were markedly lower, clinical, bacteriologic, and histopathologic observations in chickens treated with dexamethasone were similar to those previously found in surgically thymectomized chickens. These results may support the theory that multiple synovitis of chickens caused by M synoviae infection develops mainly because of an immune response, especially by thymus-dependent functions.