Short term immobilisation (seven days) of an antigen induced arthritic knee joint of the mouse led to attachment of polymorphonuclear leucocytes to the cartilage surface and severe cartilage damage. The events at the patellar cartilage surface were studied by scanning electron microscopy and compared with the changes found in non-immobilised arthritic joints. At day 7 the immobilised arthritic joint showed an articular cartilage which was covered by a mat of fibrinous material and cell remnants. Moreover, inflammatory cells, clearly attached to the cartilage, showed large protrusions of their membranes. The cartilage surface, furthermore, showed numerous fissures of varying size and depth. These cartilage changes were much more pronounced in immobilised arthritic joints than in non-immobilised arthritic joints, and clear cell adhesion was exclusively found in the former. These findings show that immobilisation of inflamed mouse knee joints leads to increased damage and thus although clinical immobilisation is used to relieve pain, this may also risk damage to the articular cartilage.