Collagen arthritis can be induced readily in many strains of rats by immunizing them with heterologous or homologous native type II collagen emulsified in incomplete Freund's adjuvant (1). This disease, which is characterized by the development of both cellular and humoral immune response to type II collagen (2-4), can be passively transferred by sensitized spleen and lymph node cells (5) as well as IgG antibodies to type II collagen (6). These findings are consistent with the proposal that collagen arthritis is the result of immunologic hypersensitivity to type II collagen. In a recent report, Trentham et al. (7) showed that rats with adjuvant arthritis exhibited both humoral and cellular sensitivities to homologous type II collagen, and suggested that an autoimmune response to type I! collagen is a common feature to both collagen arthritis and adjuvant arthritis. However, subsequent studies (8, 9) have produced conflicting results that shed some doubt on this exciting concept, and the discrepancies need further clarification. The present study was intended to determine if an autoimmune response to type II collagen is responsible for the induction of both collagen arthritis and adjuvant arthritis by using a relatively new immunosuppressive agent, cyclosporin, which has been shown to be capable of inducing specific immunologic tolerance in several in vivo and in vitro conditions (10, 1 I).