Frozen sections of articular cartilage, obtained from patients with rheumatoid arthritis (RA) or osteoarthritis (OA) undergoing joint replacement, were stained with fluoresceinated specific antisera to IgG, IgM, IgA, C1q, C4, and C3. Specimens positive for IgG were examined for IgG subclasses using mouse monoclonal antibodies. IgG was present in 22 of 34 cartilage specimens obtained from patients with RA, and in 14 of these 22 patients, a granular pattern was present. IgM, IgA, C1q, and C3 when present showed a similar granular pattern. In articular cartilage of patients with RA, all IgG subclasses tended to be present. The remaining eight specimens positive for IgG from patients with RA had staining patterns also seen in patients with OA. IgG staining was present in 31 of 117 cartilage specimens obtained from patients with OA and none had the granular pattern seen in RA. Intermittent linear staining at the surface was the most common pattern seen in cartilage from patients with OA. The different patterns of immune deposits in articular cartilage in RA and OA suggest that antibodies with different specificities are present or that different mechanisms of immune deposit formation exist in these disorders.