Spinal cord sections from mice injected with Theiler's murine encephalomyelitis virus (TMEV) and surviving for 1 year and longer after infection were stained for virus antigen by two immunohistochemical techniques. Virus antigen was detected from 1 to 2½ years after infection, a time when no virus was recovered at an assay sensitivity of 50 plaqueforming units per gram of tissue. The implication this has regarding the detection of a virus in MS is discussed.