Serum antibodies against native collagen type II and cartilage proteoglycans as well as proliferative responses of spleen lymphocytes to these antigens were evidenced during the progression of antigen-induced arthritis in mice. Because there were some correlations to the severity of arthritis, it is suggested that autoimmunity to cartilage constituents play a role for the persistence of the joint inflammation and cartilage destruction in this experimental model of chronic arthritis.