Septic arthritis in one knee joint was produced in 18 rabbits, the contralateral knee being left intact. Sections stained with fluorescein conjugated anti-IgG and with fluorescein conjugated anti-IgM showed layers of immunoglobulins on the articular cartilage in 16 out of 17 injected joints, and 10 out of 17 supposedly "normal" knee joints. Concurrent tests for circulating anti-staphylococcal alpha-lysin gave negative results. Cultures of synovial fluids were negative. These observations suggest that articular cartilage may act as an auto-antigen; immunological mechanisms involved may be due to the combined activity of both cell-bound and humoral antibodies. There are 2 components to a septic arthritis, the primary being a bacterial and the secondary immunologic in nature.