1. Study of 200 cases investigated by pneumarthrography has shown that the introduction of air or oxygen into the knee joint is followed by a specific reaction characterised by transient localised eosinophilia in synovial membrane and synovial fluid.

2. Complications after meniscectomy seem to be more frequent when operation is performed during the phase of eosinophil reaction.

3. The significance of these observations has been discussed in relation to other causes ofeosinophilia and it is suggested that this method of local production of eosinophils may be of experimental importance.