PL/J mice developed chronic relapsing experimental allergic encephalomyelitis (EAE) after receiving syngeneic guinea pig basic protein (GPBP)-immune lymph node cells or spleen cells which were cultured in the presence of GPBP or the encephalitogenic N-terminal peptide of GPBP. The presence of L3T4+ cells and in vitro proliferation in response to GPBP are required for the successful transfer. Pathologically, passively transferred EAE in the virus-free PL/J strain was characterized by an infiltration in the central nervous system by small lymphocytes, followed by the appearance of macrophages, and subsequently by primary demyelination. These findings were similar to those previously observed in chronic relapsing EAE in SJL/J mice. However, in some experiments pathologic examination of the spinal cord showed large demyelinating lesions which were necrotic and infiltrated with eosinophilic polymorphonuclear leukocytes. Sera from mice with this pathology contained antibodies to murine hepatitis virus and extensive search identified a few areas of coronavirus replication. The pathology of autoimmune mediated demyelination may be altered in the presence of coronavirus infection but the clinical pattern of EAE expression did not differ between virus-free and coronavirus-infected mice.