We investigated the immunolocalization of S100A8 and S100A9 in sections of antigen-induced arthritis (AIA); the effect of IL-1α on S100a8 and S100a9 expression and immunolocalization in mouse cartilage explants in vitro; the in vivo expression and immunolocalization of S100A8 and S100A9 in cartilage during progressive cartilage destruction in an OA compared with an inflammatory arthritis model in mice; and the effect of S100A8 and S100A9 on the expression by primary adult ovine articular chondrocytes of key extracellular matrix molecules, matrix degrading enzymes, and their inhibitors. Here, S100A9 is linked to arthritic joint disease.