Intravenous injection of Mycoplasma bovis produced in calves arthritis with synovial infiltration of lymphocytes, macrophages and neutrophils. Necrosis was observed focally around blood vessels. Joint spaces contained fibrinopurulent exudate. Parenterally vaccinated calves had a markedly reduced frequency of arthritic joints. Immunoglobulin classes and specific antibody in joint fluids were quantitatively less than in sera but significantly greater in arthritic than in normal joints. The possible mechanisms of induction of joint fluid antibody are discussed.