Autoimmunity to collagen was investigated in several naturally occurring arthropathies of the dog. Increased levels of serum anti-native collagen type II antibody, as assessed by ELISA, were shown in 72.4% of dogs with rheumatoid arthritis (RA), 88% of dogs with infective arthritis (IA) and 52% of dogs with osteoarthritis (OA) (p less than 0.001). The mean levels of antibody in cruciate disease patients (CR) were also significantly increased compared to control dogs (p less than 0.01). Serum anti-collagen antibody in OA dogs correlated with that in precipitated serum immune complexes. There was also a correlation between anti-collagen antibody level in synovial fluid and in synovial fluid complexes in dogs with rupture of the cranial cruciate ligament. In all patient groups, collagenase digestion of polyethylene glycol (PEG) precipitates from sera and synovial fluids caused a significant rise in specific antibody levels to collagen, indicating the presence of collagen-anti-collagen complexes in all arthropathies. In dogs with RA, the levels of collagen-specific antibody in synovial fluid complexes correlated with the total IgG in these complexes. These findings implicate collagen-anti-collagen complexes in the pathogenesis of naturally occurring joint diseases in the dog, but they are unlikely to be the primary aetiological mechanism.