15-Deoxyspergualin (DSG), a novel antitumor antibiotic, was tested for treatment of acute experimental allergic encephalomyelitis (EAE) in Lewis rats. Clinical and histologic signs of EAE by active sensitization with myelin basic protein were profoundly inhibited by prophylactic administration of DSG in a dose-dependent manner. By the treatment during the inductive phase, the onset of EAE was significantly delayed. Antigen-specific proliferation of lymph node cells and the ability of spleen cells to transfer EAE were suppressed but concanavalin A-induced lymphocyte proliferation was not altered. Passive EAE induced with an encephalitogenic T cell line was also prevented by DSG-treatment, although DSG did not suppress but rather augmented the activation of T cells in vitro. Taken together, DSG is not a non-specific lymphocyte toxin but a unique immunomodulator that can suppress both inductive and effector phases of EAE.