Lethally irradiated Lewis (LEW) rats, reconstituted with syngeneic bone marrow and next given Cyclosporin A (CyA) for several weeks, develop disease (Cyclosporin A-induced autoimmunity; CyA-AI) after withdrawal of CyA. This disease resembles in terms of dermal changes the acute dermatitis and chronic scleroderma also seen in graft-versus-host disease (GVHD). In this study we report the relative resistance of the Brown Norway (BN) rat strain to the induction of CyA-AI. In contrast to LEW rats, in which CyA-AI was originally described, BN rats showed no acute dermatitis or scleroderma-like skin pathology in spite of comparable changes in the thymus and a maturation arrest of CD4+ T cells. The difference was also demonstrated functionally for whereas in LEW rats delayed-type hypersensitivity (DTH) reactions' could not be elicited during CyA-AI, these were within normal limits in BN rats subjected to the same protocol; NK activity on the other hand was unaffected in both strains. The observation that BN rats developed very mild late disease as evidenced by a slight though significant weight loss suggests that the BN strain is susceptible to the disease but that lesser effector cell generation or, alternatively, stronger suppressor cell responses may prevent dermal disease. These observations may contribute to the elucidation of the mechanisms involved in this experimental autoimmune disease.