This paper describes alternative ways of deriving lines of rat lymphocytes capable of mediating EAE. The cells were demonstrated to bear markers specific for T lymphocytes and to be negative for surface Ig, and for Fc and complement receptors. The line cells were found to proliferate in vitro specifically in response to myelin basic protein (BP) associated with accessory cells syngeneic at the major histocompatibility complex (MHC). This accessory cell function could not be provided by purified populations of adherent cells from spleen, peritoneal exudates, or bone marrow cultures alone.