Serum autoantibodies to cartilage cell surface proteins have been detected in the blood from patients with osteoarthritis (OA) and rheumatoid arthritis (RA). As tested by ELISA using plasma membrane vesicles as a substrate, patients with RA exhibited significantly elevated antibody titers, whereas titers were somewhat lower in OA, and only marginal in controls. The antibodies did not crossreact with plasma membranes from fibroblasts or epithelial cells, but showed interspecies crossreactivity towards chicken and rat cartilage cell surface proteins. A group of at least 9 distinct immunoreactive polypeptides could be characterized by Western blot analysis. These peptides had molecular weights of Mr28, 30, 38, 42, 66, 76, 78, 116, and 155 kD. The patients each expressed an individual pattern of reactive antibodies against these proteins. Autoantibody reactivity to cell surface associated collagen type II was also observed.