To define autoantibody specificities of immune complexes sequestered in articular cartilage of patients with rheumatoid arthritis and osteoarthritis, extracts were obtained from articular cartilage specimens from 16 patients with rheumatoid arthritis, 11 patients with osteoarthritis, and 6 normal controls. Radioimmunoassays of the extracts revealed that rheumatoid cartilage contained 37 times more IgM and 14 times more IgG than did normal cartilage extracts. In addition, osteoarthritic cartilage contained 3 times more IgM and IgG than the normal tissues. IgM rheumatoid factor was found in 13 of 16 rheumatoid cartilage extracts but in none of 11 osteoarthritic or 6 normal control extracts. IgG rheumatoid factor was detected in 4 of 7 seropositive rheumatoid but in none of 5 osteoarthritic cartilage extracts. More than 60% of the rheumatoid cartilage extracts were positive for native and denatured collagen II antibodies. Surprisingly, 50% of the osteoarthritic specimens also contained significant titers of collagen antibodies. Similar results were obtained with osteoarthritic menisci extracts. These findings indicate that the immune complexes sequestered in rheumatoid cartilage contain autoantibodies that are probably synthesized locally by cells infiltrating the inflamed synovium. If immune complexes trapped in cartilage play an important role in cartilage damage, our findings would provide a possible pathogenic mechanism that explains the self-perpetuating and chronic nature of cartilage degradation in rheumatoid arthritis and osteoarthritis.