We have raised a monoclonal antibody against cultured human osteoblast-like cells, which recognises a novel matrix antigen with a unique distribution in skeletal tissue. Immunocytochemistry on cryostat sections of normal human articular cartilage shows the antigen restricted to a zone immediately adjacent to the articular surface. In cartilage with evidence of degeneration, staining was observed in the territorial matrix of chondrocytes. There was intense staining throughout the matrix in samples from patients with marked osteoarthritic lesions. The antigen has been found in cartilage, the human breast cancer cell line Hs 578T and osteoclastoma tissue. On immunoscreening of an osteoclastoma cDNA library constructed in lambda gt11, a clone of 0.7kb was identified and sequenced but had no homology to sequences in the EMBL/Genbank database. Northern analysis identified a 1.8kb signal in osteoclastoma mRNA.In order to isolate the antigen, we dissected articular cartilage from the femoral head of patients undergoing hip replacement surgery. Ground cartilage was sequentially washed with Tris-EDTA (TE), (TE)+Tween then (TE)+SDS in the presence of protease inhibitors, to obtain SDS fractions containing the antigen. Subsequent Western blot analysis identified a high molecular weight component >400k, which on reduction yielded a number of discrete components ranging in size from 60k to 190k. This observation is consistent with the antigen being part of a large disulphide linked complex. Moreover, the antigenicity observed on Western blots correlated with the degree of matrix destruction observed at the articular surface. Characterisation of this antigen may provide an insight into the mechanisms of cartilage destruction observed in many joint pathologies.