The purpose of this study was to determine the immunolocalization of glycosaminoglycans (GAGs) in the mandibular condylar cartilage of senescence-accelerated mice (SAM) with early osteoarthritis (OA). The cartilages in the mandibular condyles and in the proximal tibial epiphyses from two 10-month-old SAM strains, SAMP8 and SAMR1 (a control), were examined for the immunolocalization of chondroitin-4-sulphate (C4S) and keratan sulphate (KS). Morphologically, the mandibular condylar cartilage was disrupted with clefts in SAMP8 but not in SAMR1. C4S was distributed throughout the two cartilages. In contrast, KS was located in extracellular regions in the fibrous and proliferative cell layers of mandibular cartilage and in the lower maturative cell layer of epiphyseal cartilage. The two GAGs, particularly KS, were intensely immunostained around the clefts. These results suggest that C4S and KS could play differential roles in the structural function of articular cartilage and their functions might be involved in early mandibular OA lesions.