Acute experimental allergic encephalomyelitis (EAE) was induced in C57BL/6J and SJL/J mice by injection of isologous spinal cord homogenate given in conjunction with Bordetella pertussis and Freund's adjuvant. SJL/J mice showed a highly aggressive and 100% lethal form of the disease; C57BL/6J mice were much less susceptible as they had low morbidity rates (20 to 40%), low disease scores, and mostly no mortality. Treatment of these low susceptibility mice with neutralizing mAb against IFN-gamma caused an increase in morbidity rates as well as significant mortality (up to 80%). Similar antibody treatment did not affect the course of the disease in the high susceptibility SJL/J mice. However, treatment of these mice with IFN-gamma resulted in reduced morbidity and mortality. A similar but less pronounced inhibition of the disease in SJL/J mice could be obtained by administration of IFN-alpha/beta or by acute infection with lactate dehydrogenase virus. The results indicate that endogenous as well as exogenous IFN can exert a down-regulating effect on the development of EAE. They also indicate that endogenous IFN-gamma is produced during the development of EAE and plays a disease-limiting role.