Mycoplasma synoviae was isolated from trachea of a chicken with airsacculitis and was designated as strain H. When the isolate was administered by aerosol into chickens mild airsacculitis occurred, and the organisms were recovered mainly from trachea, but rarely from leg joints. By inoculation into leg joint or foot pad, injected sites were swollen, even though the organisms were not recovered 6 weeks after inoculation. The strain H and two strains of known M. synoviae, strains WVU 1853 and F2-C AS, produced airsacculitis and synovitis at various degrees after aerosol and leg joint inoculations. Synovitis seemed to be predominant findings caused by strain WVU 1853, whereas mild airsacculitis combined to synovitis was caused with strains F2-C AS and H.