Polyarthritis in rats could be induced by intraperitoneal infection with M. arthritidis. The mycoplasma culture alone or in combination with infusorial earth was used as inoculum. The histopathological changes consisted of initial lesions in the transitional zone of the articular capsule which could be attributed to disturbances of vascular permeability. After two days suppurative arthritis occurred, which was accompanied by massive inflammatory lesions of the articular capsule and of the periarticular tissues. Towards the end of the observation period erosions of cartilage, pannus formation and periarticular foci of purulent necrobiosis were found. Especially the synovial membrane was subjected to profound changes and dense cellular infiltration. The cell infiltration, mesenchymal proliferation and changes of vascular permeability showed variations in timing and degree depending on the modi of infection. The histological examinations of organs indicated generalization of the infection.