The central role of T lymphocytes in the initiation, regulation and propagation of autoimmune diseases defines them as most suitable targets for selective immunotherapy. The recent advance in culturing human and animal T cell lines allows us to select monoclonal antibodies specific for differentiation antigens expressed by activated T lymphocytes. We selected a monoclonal antibody cytotoxic for a subpopulation of activated rat T cells. In vivo, this antibody effectively blocks immune responses to foreign antigens or autoantigen and prevents development of autoimmune diseases like experimental allergic encephalomyelitis and adjuvant arthritis. Even already established disease can be blocked by a single injection of antibody. Furthermore, this monoclonal antibody can be used to monitor the course of autoimmune disease progression from peripheral blood samples.