The mode of action of the immunosuppressant mitoxantrone was examined in murine models of demyelinating disease. The drug has been shown to block antigen induced proliferative activity and to inhibit myelin degradation by leucocytes from paralysed mice. Mitoxantrone blocked myelin breakdown by macrophages although phagocytosis was not affected. Further evidence was obtained to indicate that mitoxantrone acts therapeutically in reducing, or at high dose, preventing signs of EAE developing in mice immunized with spinal cord homogenate and Freund's complete adjuvant. Mitoxantrone also significantly inhibited the incidence of relapse when treatment was initiated during the post-acute remission period.