Experimental allergic encephalomyelitis (EAE) was produced in 2-to 24-mo.-old F344 rats by active immunization and by passive transfer of lymphoid cells (adoptive immunization) from young rats. With both methods, some 24-mo.-old rats had a reduced, ability to develop this autoimmune disease. However, the immunological deficit was related to incidental serious diseases rather than to old age per se. The aged F344 rat is not a suitable model for comparing EAE to multiple sclerosis in man because the decreased incidence of multiple sclerosis after the age of 50 is not related to other serious illnesses.