This study set out to investigate the in vivo expression and distribution of the porcine homologues of the intercellular adhesion molecule-1 (ICAM-1) and MHC Class II as markers of chondrocyte activation during the development of chronic polyarthritis, which was experimentally induced in Landrace pigs by intra-articular injection of Erysipelothrix rhusiopathiae. ICAM-1 was found to be strongly expressed in vivo on chondrocytes and synovial cells in arthritic joints but was not detected in cartilage from unaffected joints. Although the majority of ICAM-1 positive chondrocytes did not co-express MHC Class II, chondrocyte-type cells expressing both molecules were detected in the transition zone as the disease progressed, particularly at 5 months post-infection. At this stage infiltration of CD4+ T lymphocytes into the damaged cartilage was also apparent. ICAM-1 and MHC Class II are not constitutively expressed on porcine chondrocytes but appeared to be induced as arthritis progressed. The detection of these markers in the pig helps to establish the validity of this animal model for immunopathological studies.