Human synovium was analyzed for the possible expression of cartilage oligomeric matrix protein (COMP). Immunostaining with polyclonal antiserum to COMP demonstrated positive staining within the synovial cells and immediately subjacent connective tissue, with less intense staining in the deeper connective tissue. Western blot analysis using either polyclonal or monoclonal antibodies to human COMP confirmed the presence of COMP by immunoreactive bands with the same molecular mass (approximately 110 kDa) as purified articular cartilage COMP. PCR using oligonucleotides that span human COMP exons 7-13 revealed identical amplification products from cDNA prepared from either human chondrocytes or synovium. Northern blot analysis using a biotinylated-probe to human COMP, spanning exons 12-13, also reveal an identical hybridization product to either human chondrocyte or synovium total RNA. Human synovium should be considered as a potential tissue source of COMP in any investigation of biological markers of cartilage metabolism.